DialogService

public class MainViewModel : ViewModelBase {
    protected IDialogService DialogService { get { return this.GetService<IDialogService>(); } }
    // ...
}

Public Class MainViewModel
    Protected ReadOnly Property DialogService() As IDialogService
        Get
            Return Me.GetService(Of IDialogService)()
        End Get
    End Property
    ' ...
End Class

<UserControl x:Class="MVVMDemo.Services.DialogServiceView" 
            ...
            DataContext="{dxmvvm:ViewModelSource local:DialogServiceViewModel}">
    <UserControl.Resources>
        <DataTemplate x:Key="dialogTemplate">
            <dxlc:LayoutControl Orientation="Vertical" VerticalAlignment="Top" MinWidth="300">
                <dxlc:LayoutItem Label="Customer Name:">
                    <dxe:TextEdit Text="{Binding CustomerName, UpdateSourceTrigger=PropertyChanged}"/>
                </dxlc:LayoutItem>
            </dxlc:LayoutControl>
        </DataTemplate>
    </UserControl.Resources>
    <dxmvvm:Interaction.Behaviors>
        <!-- The ViewTemplate property specifies the dialog template -->
        <dx:DialogService x:Name="DialogService" ViewTemplate="{StaticResource dialogTemplate}">
            <dx:DialogService.DialogStyle>
                <Style TargetType="Window">
                    <Setter Property="SizeToContent" Value="WidthAndHeight" />
                    <Setter Property="WindowStyle" Value="ToolWindow" />
                </Style>
            </dx:DialogService.DialogStyle>
        </dx:DialogService>
    </dxmvvm:Interaction.Behaviors>
    ...
</UserControl>

public static class DialogServiceExtensions {
    // Use these methods when the a child View is defined though the DialogService.ViewTemplate or DialogService.ViewTemplateSelector property. 
    //The child View should not contain a View Model, because it is passed through the service.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, object viewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, object viewModel);

    // Use these methods to create a child View and pass the specified View Model to the created child View.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, string documentType, object viewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, string documentType, object viewModel);

    // Use these methods to create a child View that already contains a View Model. 
    // The Parameter and ParentViewModel are passed to the child View Model.
    public static UICommand ShowDialog(this IDialogService service, IEnumerable<UICommand> dialogCommands, string title, string documentType, object parameter, object parentViewModel);
    public static MessageResult ShowDialog(this IDialogService service, MessageButton dialogButtons, string title, string documentType, object parameter, object parentViewModel);
}

Public NotInheritable Class DialogServiceExtensions
    ' Use these methods when the a child View is defined though the DialogService.ViewTemplate or DialogService.ViewTemplateSelector property. 
    ' The child View should not contain a View Model, because it is passed through the service.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, viewModel As Object) As UICommand
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, viewModel As Object) As MessageResult

    ' Use these methods to create a child View and pass the specified View Model to the created child View.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, documentType As String, viewModel As Object) As UICommand
    System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, documentType As String, viewModel As Object) As MessageResult

    ' Use these methods to create a child View that already contains a View Model. 
    ' The Parameter and ParentViewModel are passed to the child View Model.
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogCommands As IEnumerable(Of UICommand), title As String, documentType As String, parameter As Object, parentViewModel As Object) As UICommand
    <System.Runtime.CompilerServices.Extension> _
    Public Shared Function ShowDialog(service As IDialogService, dialogButtons As MessageButton, title As String, documentType As String, parameter As Object, parentViewModel As Object) As MessageResult
End Class

using DevExpress.Mvvm;
using DevExpress.Mvvm.POCO;

namespace MVVMDemo.Services {
    public class DialogServiceViewModel {
        public void ShowDetail(string serviceName) {
            DialogServiceDetailViewModel detailViewModel = DialogServiceDetailViewModel.Create();

            UICommand registerCommand = new UICommand() {
                Caption = "Register",
                IsDefault = true,
                Command = new DelegateCommand(
                    () => { }, 
                    () => !string.IsNullOrEmpty(detailViewModel.CustomerName)
                )
            };
            UICommand cancelCommand = new UICommand() {
                Caption = "Cancel",
                IsCancel = true,
            };

            IDialogService service = this.GetService<IDialogService>(serviceName);
            UICommand result = service.ShowDialog(
                dialogCommands: new[] { registerCommand, cancelCommand }, 
                title: "Registration Dialog", 
                viewModel: detailViewModel
            );

            if(result == registerCommand)
                MessageBox.Show("Registered: " + detailViewModel.CustomerName);
        }
    }
}

Imports DevExpress.Mvvm
Imports DevExpress.Mvvm.POCO

Namespace MVVMDemo.Services
    Public Class DialogServiceViewModel
        Public Sub ShowDetail(ByVal serviceName As String)
            Dim detailViewModel As DialogServiceDetailViewModel = DialogServiceDetailViewModel.Create()

            Dim registerCommand As New UICommand() With {
                .Caption = "Register",
                .IsDefault = True,
                .Command = New DelegateCommand(
                    Sub()
                    End Sub,
                    Function() (
                    Not String.IsNullOrEmpty(detailViewModel.CustomerName)
                    ))
            }
            Dim cancelCommand As New UICommand() With {
                .Caption = "Cancel",
                .IsCancel = True
            }

            Dim service As IDialogService = Me.GetService(Of IDialogService)(serviceName)
            Dim result As UICommand = service.ShowDialog(
                dialogCommands:={registerCommand, cancelCommand},
                title:="Registration Dialog",
                viewModel:=detailViewModel
            )

            If result Is registerCommand Then
                MessageBox.Show("Registered: " & detailViewModel.CustomerName)
            End If
        End Sub
    End Class
End Namespace

<UserControl ...
             xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
             xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm">
    <Grid>
        <dxmvvm:Interaction.Behaviors>
            <dxmvvm:DialogService SetWindowOwner="True"
                                  WindowStartupLocation="CenterOwner">
                <dxmvvm:DialogService.DialogStyle>
                    <Style TargetType="dx:ThemedWindow">
                        <Setter Property="Width" Value="500" />
                        <Setter Property="Height" Value="300" />
                    </Style>
                </dxmvvm:DialogService.DialogStyle>
            </dxmvvm:DialogService>
        </dxmvvm:Interaction.Behaviors>
        ...
    </Grid>
</UserControl>

<UserControl x:Class="Example.View.MainView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"             
             xmlns:dx="http://schemas.devexpress.com/winfx/2008/xaml/core"
             xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm"
             xmlns:ViewModel="clr-namespace:Example.ViewModel"
             xmlns:View="clr-namespace:Example.View"
             DataContext="{dxmvvm:ViewModelSource ViewModel:MainViewModel}">

    <dxmvvm:Interaction.Behaviors>
        <dx:DialogService DialogWindowStartupLocation="CenterOwner">
            <dx:DialogService.ViewTemplate>
                <DataTemplate>
                    <View:RegistrationView />
                </DataTemplate>
            </dx:DialogService.ViewTemplate>
            <dx:DialogService.DialogStyle>
                <Style TargetType="dx:ThemedWindow">
                    <Setter Property="Width" Value="300" />
                    <Setter Property="Height" Value="160" />
                </Style>
            </dx:DialogService.DialogStyle>
        </dx:DialogService>
    </dxmvvm:Interaction.Behaviors>

    <Grid x:Name="LayoutRoot" Background="White">
        <Button HorizontalAlignment="Center"
                VerticalAlignment="Center"
                Command="{Binding ShowRegistrationFormCommand}"
                Content="Show Registration Form" />
    </Grid>

</UserControl>

namespace Example.ViewModel {
    public class RegistrationViewModel {
        public virtual string UserName { get; set; }
    }
}

<UserControl x:Class="Example.View.RegistrationView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"                         
             xmlns:dxmvvm="http://schemas.devexpress.com/winfx/2008/xaml/mvvm">
        <StackPanel HorizontalAlignment="Center"
                VerticalAlignment="Center"
                Orientation="Horizontal">
        <TextBlock VerticalAlignment="Center" Text="User Name: " />
        <TextBox Width="200" Text="{Binding UserName, UpdateSourceTrigger=PropertyChanged}">
            <dxmvvm:Interaction.Behaviors>
                <dxmvvm:FocusBehavior />
            </dxmvvm:Interaction.Behaviors>
        </TextBox>
    </StackPanel>

</UserControl>

using DevExpress.Mvvm;
using DevExpress.Mvvm.POCO;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Windows;

namespace Example.ViewModel {
public class MainViewModel {
    protected IDialogService DialogService { get { return this.GetService<IDialogService>(); } }
    RegistrationViewModel registrationViewModel = null;
    public void ShowRegistrationForm() {
        if(registrationViewModel == null)
            registrationViewModel = new RegistrationViewModel();

        UICommand registerCommand = new UICommand() {
            Caption = "Register",
            IsCancel = false,
            IsDefault = true,
            Command = new DelegateCommand<CancelEventArgs>(
                x => myExecuteMethod(),
                x => !string.IsNullOrEmpty(registrationViewModel.UserName)),
        };

        UICommand cancelCommand = new UICommand() {
            Id = MessageBoxResult.Cancel,
            Caption = "Cancel",
            IsCancel = true,
            IsDefault = false,
        };

        UICommand result = DialogService.ShowDialog(
            dialogCommands: new List<UICommand>() { registerCommand, cancelCommand },
            title: "Registration Dialog",
            viewModel: registrationViewModel);

        if (result == registerCommand) {
            //...
        }
    }

    private void myExecuteMethod() {
        Debug.Print("Registration complete");
    }
}
}

Namespace Example.ViewModel
    Public Class RegistrationViewModel
        Public Overridable Property UserName() As String
    End Class
End Namespace

Imports DevExpress.Mvvm
Imports DevExpress.Mvvm.POCO
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Diagnostics
Imports System.Windows
Imports System.Windows.Input

Namespace Example.ViewModel
Public Class MainViewModel
    Protected ReadOnly Property DialogService() As IDialogService
        Get
            Return Me.GetService(Of IDialogService)()
        End Get
    End Property
    Private registrationViewModel As RegistrationViewModel = Nothing
    Public Sub ShowRegistrationForm()
        If registrationViewModel Is Nothing Then
            registrationViewModel = New RegistrationViewModel()
        End If

        Dim registerCommand As New UICommand() With {.Caption = "Register", .IsCancel = False, .IsDefault = True, .Command = New DelegateCommand(Of CancelEventArgs)(Sub(x) myExecuteMethod(), Function(x) (Not String.IsNullOrEmpty(registrationViewModel.UserName)))}

        Dim cancelCommand As New UICommand() With {.Id = MessageBoxResult.Cancel, .Caption = "Cancel", .IsCancel = True, .IsDefault = False}

        Dim result As UICommand = DialogService.ShowDialog(dialogCommands:= New List(Of UICommand)() From {registerCommand, cancelCommand}, title:= "Registration Dialog", viewModel:= registrationViewModel)

        If result Is registerCommand Then
            '...
        End If
    End Sub

    Private Sub myExecuteMethod()
        Debug.Print("Registration complete")
    End Sub
End Class
End Namespace