I would (and have) make a new xaml for your RadWindow, in doing this you can create its own VM and the properties you need to hand the data to it.
(this is the DetailsWindow.xaml)
<!-- make it look as desired -->
then in the codebehind (DetailsWindow.xaml.cs) something like
DetailsWindow : RadWindow
Then if you create a new DetailsWindow you can do DetailsWindow.SomeIncomingItem = foo. So your button click could create a new DetailsWindow and set the value. This doesn't really maintain MVVM and is merely one possibility. Technically your RadWindow is another V and should have a VM. Your main V could trigger your VM through a command to make the VM for the RadWindow which fires an event that your V is listening to and passes in the VM for the RadWindow. The event then creates a RadWindow with the VM backing it.
Does that make sense?