Since the patch that came with the Art of War expansion, it is possible to start wars to reconquest core provinces from enemies that belong to your vassal.
However, when I finally win and capture those provinces, I have no option to return those provinces to my vassal. Instead I can only annex them myself. I can sell them to my vassal afterwards, but I get unnecessary AE this way for provinces I never wanted for myself in the first place.
Is there a way to return those core provinces directly to my vassal in peace negotiations? If yes, under which conditions will this option show up?