I might be wrong with this, but I think the way it went down was that when Paul Bettany was casted as the AI “butler” J.A.R.V.I.S., the producers had not jet in mind him to become Vision, rather they made that up along the way. Had they stayed true to Vision’s comic book origin, then there would have been a casting directly for Vision, which might have gone very differently.
If that’s what you think, then you should have a look at the comic’s Ultimates Universe version of Wanda and Pietro – and then you will never get those images out of your brain