Add SelectedItemsSource property which can be bound to a collection in the view model. In this case when the RadListBox is in Multiple/Extender mode, the selection can be manipulate from the ViewModel.
Available in the 2015 Q1.
Fix it for WPF as well.
Please also add support for using SelectedItem (the single selected item property) from code-behind in a MVVM scenario in order to change the currently selected item on the UI.
Changing the type of the SelectedItems to ObservableCollection<object> like the radgrid.