All CM Punk needs to complete his heel persona is the Majin sign on his forehead...oh...and Vegeta's english accent :L. In all seriousness though, even though Punk has defeated Cena clean, Cena is still shown as the "main character" and essentially the strongest, the rock wanted to face John Cena, not CM punk. CM punk still feels like hes being overshadowed so the Vegeta angle can still work.
Also according to Vince, the casual audience has a memory span of 3 weeks so its all good
