Please refer to How can I tell my DataTemplate to bind to a property in the PARENT ViewModel?
I have similar problem... But this solution didn't work for me. I have a MainViewModel which has an observable collection of another view model say View1/ViewModel1. This view has a tree control and I need context menu for the tree. My main view has a menu. Those main menu and context menu are connected. So how can I bind the context menu commands to the main viewmodel's properties?