Hayden needs the work. And he's been in the suit before. It's a physical acting role, and not one where Hayden would have to voice dialogue. That's James Earl Jones' job.
Now if he's being cast to play a Force Spirit Anakin Skywalker in Episode 8? Then I can appreciate some of the concerns people might have. But if Anakin as a Force Spirit is integral to the story? You can certainly speculate however you want. From the trailers it seems that someone in the Sequels is a devotee to Darth Vader, seeing how the melted Vader helmet & mask are set up as an object of worship. Recasting the Anakin role would damage any sense of continuity
Couldn't they just make a digital version of Hayden as a force ghost instead of actually hiring him? It would be a lot more realistic.
Since Force ghosts are immortal in movie cannon (the in five years it gets too hard like in the Thrawn trilogy was taken out in ROTS), I fully expect to see Yoda, Anakin and Obi-Wan as force ghosts.