WheresBlackhawk said:Obiwan would sense who Vader is and Vader's response helps to seal the deal: Obi asks," YOU are Lord Vader?" Vader should attack him ferociously. Obi starts,"...Anakin..." Vader finally speaks, cutting him off, "is no more." The audience whould be convinced the other student survived.
Love this idea!!
I'm doing some brainstorming myself. More later.