Besides, there are ways to return the character to life without resurrectting him
Darth Vader, likes Boba Fett, is hard to separate from his armor.
It would be very Star Wars like to have a Dark Jedi use exactly the same armor as Darth Vader to terrorize the republic.
I like, I like. To be fair though, I'm not sure we can say anything at all about these films - I mean is there any substantial evidence that says the sequels will occur after the events of ROTJ, or that they will be sequels at all?
EDIT: Then again, the fact that its called Episode VII seems to imply it will be a sequel...
New armor can be made. And a seen where this is happening would explain to viewers that this is not Darth Vader but a pretender - still, it could confuse the general audience.