Vin Diesel has been posting some odd stuff on his Facebook page. The pictures are down now, but take a look at this screenshot.
There is nothing to confirm at this point. Avengers 2 is still three years off. No casting, no characters. Hell there might not even be a script yet. But Vin Diesel would be a pretty cool Vision.
He was previously rumored to play Silver Surfer. As we know that did not happen.