public interface IModule
{
    /// <summary>
    /// Название выводимое в меню
    /// </summary>
    string Name { get; }
    /// <summary>
    /// Ссылка на вьюшку
    /// </summary>
    UserControl UserInterface { get; }
    /// <summary>
    /// Отключение текущей вьюшки, для вьюмоделей с реализацией IDisposable
    /// </summary>
    void Deactivate();
}

class SeparateDemo : IModule
{
    //наша неразлучная парочка
    private SeparateDemoView view;
    private SeparateDemoViewModel viewModel;

    public string Name => "Вьюшка SeparateDemo";


    public UserControl UserInterface
    {
        get { if (view == null) CreateView(); return view; }
    }

    private void CreateView()
    {
        //загружаем вьюшку
        view = new SeparateDemoView();
        //её вьюмодель
        viewModel = new SeparateDemoViewModel();
        //связываем их между собой
        view.DataContext = viewModel;
    }

    public void Deactivate()
    {
        viewModel.Dispose();
        view = null;
    }
}

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="180" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>
    <ListBox Grid.Column="0"
             ItemsSource="{Binding Modules}"
             SelectedItem="{Binding SelectedModule, Mode=TwoWay}">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Name}" />
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>
    <ContentPresenter x:Name="contentPresenter" Grid.Column="1"
                      Content="{Binding UserInterface}" />
</Grid>

class MainWindowViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged = delegate { };

    //ctor
    public MainWindowViewModel(IEnumerable<IModule> modules)
    {
        Modules = modules.OrderBy(m => m.Name).ToList();
        if (this.Modules.Count > 0)
        {
            SelectedModule = this.Modules[0];
        }
    }

    //Properties

    /// <summary>
    /// Список всех возможных загрузчиков пар View и ViewModel
    /// </summary>
    public List<IModule> Modules { get; private set; }


    /// <summary>
    /// Выбранная и загруженная парочка View и ViewModel
    /// </summary>
    private IModule _SelectedModule;
    public IModule SelectedModule
    {
        get { return _SelectedModule; }
        set
        {
            if (value == _SelectedModule) return;
            if (_SelectedModule != null) _SelectedModule.Deactivate();
            _SelectedModule = value;
            PropertyChanged(this, new PropertyChangedEventArgs(nameof(SelectedModule)));
            PropertyChanged(this, new PropertyChangedEventArgs("UserInterface"));
        }
    }

    /// <summary>
    /// То, что отображает View через ContentPresenter
    /// </summary>
    public UserControl UserInterface
    {
        get
        {
            if (SelectedModule == null) return null;
            return SelectedModule.UserInterface;
        }
    }
}

static class ReflectionHelper
{
    public static IEnumerable<T> CreateAllInstancesOf<T>()
    {
        return typeof(ReflectionHelper).Assembly.GetTypes()
            .Where(t => typeof(T).IsAssignableFrom(t))
            .Where(t => !t.IsAbstract && t.IsClass)
            .Select(t => (T)Activator.CreateInstance(t));
    }
}

public partial class App : Application
{
    private void Application_Startup(object sender, StartupEventArgs e)
    {
        var mainWindow = new MainWindow();

        var modules = ReflectionHelper.CreateAllInstancesOf<IModule>();

        var vm = new MainWindowViewModel(modules);
        mainWindow.DataContext = vm;
        mainWindow.Closing += (s, args) => vm.SelectedModule.Deactivate();
        mainWindow.Show();
    }
}

class FirstDemo : ModuleBase
{
    public override string Name
    {
        get { return "Вьюшка FirstDemo"; }
    }

    protected override UserControl CreateViewAndViewModel()
    {
        return new FirstDemoView() { DataContext = new FirstDemoViewModel() };
    }
}