Run-time and design-time selection of a ViewModel using only xaml.

DesignAndRunTime.xaml

<Window Name="mMainView"
        x:Class="MyProject.MainView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:vm="clr-namespace:MyProject.ViewModels"
        mc:Ignorable="d"
        Title="My Project" Height="450" Width="800">
    <mc:AlternateContent>
        <mc:Choice Requires="d">
            <Window.Resources>
                <vm:MockViewModel x:Key="MainViewModel" />
            </Window.Resources>
        </mc:Choice>
        <mc:Fallback>
            <Window.Resources>
                <vm:MainViewModel x:Key="MainViewModel" />
            </Window.Resources>
        </mc:Fallback>
    </mc:AlternateContent>
    <Grid HorizontalAlignment="Stretch"
          VerticalAlignment="Stretch"
          DataContext="{StaticResource MainViewModel}">
    </Grid>
</Window>