Bettany has apparently been invited in to take part in some make-up and costume tests on the set of the film with director, Joss Whedon. With filming next month, it would seem like the perfect time to be inviting cast members along to try and get everything sorted for their roles before filming actually started.
How The Vision fits into the Marvel world is that he was a droid created by Ultron, being played by James Spader in Age of Ultron, as a weapon to destroy the people of Earth and The Avengers; but he went against his programming and became a hero. He even went on to marry Scarlett Witch in the comic books, and it has been announced that this character will be involved in the storyline this time around. Whether this will also happen in the film is yet to be seen.
It certainly seems like a decent role for Bettany to get into, but whether it will be too much of a storyline to fit into the next Avengers movie is yet to be seen. Whedon certainly did well with the first offering and the fans lapped up what he had to offer, so many are trusting him not to fail this time around.