If I understand you correctly you've created a custom RadComboBox and you like to synchronize the properties in your ViewModel bound to the SelectedItem and SelectedValue properties of the custom control. If the binding of the SelectedItem and SelectedValue properties is TwoWay binding, the RadComboBox first updates the bound properties and then raise the SelectionChanged event. In this case you could plug your synchronization logic in the setter of the SelectedItem and SelectedValue properties in your ViewModel.
Hope this helps.
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