It's clear that Vision is fulfilling Adam Warlock's role in Infinity War, only with the mind gem not the soul gem. I am still hopeful that Adam will inherit the soul gem after the events of Infinity War, but he's not going to be involved and that's OK.
What the did with Vision is similar to what they did with Ego. Ego is somewhat of an amalgamation of Ego and J'Son from the comics, in addition to making him a Celestial. They just needed something that would explain why Peter would be able to wield the infinity stone.
The ultimate goal of any of these films is to make a good film. Introducing new characters, especially as wide scale as Infinity War will be, could possibly hurt the story telling. We know who Vision is, and we know why he has the gem, that's one issue out of the way so they can deal with other plot points in the film.