MVVM + WPF + Popup = clueless

Here's what I would do, first I would separate my reusable parts into resources, and reference them using ContentControls in XAML. This applies to the Popup as well as to the Button. However I don't wish to restrict myself to having only a button, so I'll use a ContentControl as well for that:

The Popup template:

<ControlTemplate x:Key="PopupTemplate">    <Border                 Background="White"                 BorderBrush="Black"                 Padding="5"                 BorderThickness="2"                 CornerRadius="5">        <StackPanel Orientation="Vertical">            <Button Command="{Binding MyCommand}"                             CommandParameter="5">5</Button>            <Button Command="{Binding MyCommand}"                             CommandParameter="10">10</Button>            <Button Command="{Binding MyCommand}"                             CommandParameter="15">15</Button>            <Button Command="{Binding MyCommand}"                             CommandParameter="20">20</Button>        </StackPanel>    </Border></ControlTemplate>

The ContentControl template:

<ControlTemplate x:Key="MyControlTemplate" TargetType="ContentControl">    <Grid Name="MyControl">        <ContentPresenter Content="{TemplateBinding Content}"/>        <Popup Name="MyPopup" StaysOpen="True" Placement="Bottom">            <ContentControl Template="{StaticResource PopupTemplate}"/>        </Popup>    </Grid>    <ControlTemplate.Triggers>        <Trigger SourceName="MyControl"                  Property="UIElement.IsMouseOver"                  Value="True">            <Setter TargetName="MyPopup"                     Property="Popup.IsOpen"                     Value="True"/>        </Trigger>    </ControlTemplate.Triggers></ControlTemplate>

Now in the main window XAML, I would create the following:

<ContentControl Template="{StaticResource MyControlTemplate}">    <Button Content="Test"/></ContentControl>

If you have any questions, I'll be happy to answer.