Lemming42
Arcane
This "What if it took place in a single vault" discussion is funny considering concepts like that already exist in the form of Dead Money and Old World Blues.
They were parts of the overall game of Fallout: New Vegas, though. Not sure if I'd buy a Fallout game that took place entirely in a single vault.
Although:
I dont think anyone is talking about a true dungeon crawler but a RPG set in underground with freedom of exploration of Vault and caves. Imagine that world is actually in much worse state then being thought and going to the surface is out of question, the vault becomes too small and people are forced to expand to caves around vault. Imagine game like Arx Fatalis meets System Shock, where factions will be groups in vault who have different visions of how to live and expand and being part of their politics.
I like this idea. I also liked Battlespire (even if the time taken to get it to work on DOSBox was longer than the game itself), so I can see how it might work.