Abandoned and explorable vaults are certainly not the central element of the game - nor should they be - and the series is not called Vaults. There's no implication or suggestion of any kind, that "Yeah, once I load this up, I (should) get to explore abandoned vaults." So I don't really see your point there.
Well, it's the 1&2 supporters that are always saying that stuff "isn't true to the game". The vaults are a big part of the story line. So how would you stay true to the game with very few vaults? I don't think it's good to base the future design of the games, on the assumption that people have played all of them. Because a lot of people haven't. Probably the majority. There's probably going to be a lot of people that will play 4, that haven't played any of the old ones.