Besides, if Hayden returns as Anakin's Force ghost then technically we'll still see Darth Vader in the new movies sans the black suit. And the Dark Side.
I get the feeling that since George said the SW Saga was really the story of Anakin more than anybody else that we'll continue to see him in spirit form giving advice and wisdom to his son and daughter as they try to live and forge ahead in the post-Empire galaxy.
Actually, it might be interesting to pursue this angle. Maybe as a force ghost, there is still something that he needs to do to bring balance to the force--maybe we still have not learned what the balance truly is. It would be very cool if this story were dependent upon aspects of each of the previous trilogies.