You can take a look at the following movie http://channel9.msdn.com/Events/MIX/MIX11/OPN03 about how to show message box from the view model (at minute 23, there are two approaches you can choose from).
Regarding the indices. Actually in the e.Removed items and e.AddedItems you have the instances of your view model objects. Simply you should check their indices in the collection property you are binding to the RadTabControl.ItemsSource.
Hope this helps.
All the best,
the Telerik team
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