Hello Alex Martin,
If you have defined property Command of type ICommand in MenuItemViewModel, most probably you want to use it as a source of your Binding operation. Lets say you have added a CopyCommand class in your application that inherits ICommand. You can use it in your XAML in the following way:
With the lines above you are setting Command property of the MenuItemViewModel object. Now you have to set the Binding in all MenuItem Styles you need:
And that's it, you are now able to use your command.
Hopefully this helps.
the Telerik team