Thomas along with John Buscema created a "new" character who has a strikingly similar appearance to the 1940's Vision. The details of the "new" character are absolutely different than the 1940's character, but would the "new" character have existed at all if not for the original? Thomas has often said that he wanted to revive the 1940's Timely characters in some way. The obvious example of this is Avengers #97.
Do you think Thomas would have have chosen the name "Vision" for the "new" character if the original 1940's Vision didn't exist?
Do you think Thomas/Buscema would have come up with the existing visual design of the silver age Vision if the 1940's character didn't exist?
IMO the only way Thomas deserves co-creator credit is if he and Buscema had created an entirely new character with an entirely new look, origin and name. He did half of that, and basically had Buscema copy and update the original 1940's visual design of the character.