I mean, assume their powers work. Otherwise it's all a bit pointless!Depends on the magic rating of the universe. At the high end, Soth toasts Vader with a fireball and finishes him off with a Power Word Kill. In the middle, magic fails and Vader has the advantage as the Force isn't strictly magical. Below that, the Force stops working too but Soth can't be reanimated so he just drops down dead.
I mean, assume their powers work. Otherwise it's all a bit pointless!