KeyBinding on a TreeViewItem

But I would like to have the command on the ItemVM (as it is where it makes sense). Any ideas?

If TreeViewVM tracks the selected item through the SelectedItem property you can define InputBindings on TreeView and still have the commands implemented on the ItemVM:

<TreeView ItemsSource="{Binding Items}">
  <TreeView.InputBindings>
    <KeyBinding Key="F2" Command="{Binding SelectedItem.Rename}"/>
  </TreeView.InputBindings>
</TreeView>

Notice how the subproperty syntax SelectedItem.Rename is employed to use the ItemVM as the source of the binding.

Unfortunately, it is slightly tedious to bind to the selected item on a TreeView. You cannot bind directly to SelectedItem (as your XAML seems to suggest) but there exist various methods to overcome this limitation.

One simple method is to use Blend Interactivity:

<TreeView Name="treeView" ItemsSource="{Binding Items}">
  <i:Interaction.Triggers>
    <i:EventTrigger EventName="SelectedItemChanged">
      <i:InvokeCommandAction Command="{Binding SetSelectedItemCommand}" CommandParameter="{Binding SelectedItem, ElementName=treeView}" />
    </i:EventTrigger>
  </i:Interaction.Triggers>
</TreeView>

You will have to implement a SetSeletectedItemCommand on TreeViewVM that sets the property SelectedItem.

The key binding would need to be defined on the TreeViewItem, as that is the element with the focus. The problem is you cannot define key bindings using a Style, which is what you'd probably want to do here.

Here is one workaround that uses a custom attached property to add items to the InputBinding collection via a Style. So you'd want to use something like that to define your Style, which you'd assign to TreeView.ItemContainerStyle.