The control vault in the area. Normal people, who were supposed to come out and repopulate the area. Except, when they opened up their vault to trade with folks, they were discovered by the Fiends. And those who've seen what the Fiends like to do....well...Vault 3 dweller's fates aren't surprising.

Question (minor): What happened to all the bodies if they did all meet a violent end at the hands of the Fiends? Did any of the previous dwellers escape the vault?

The sacrifice vault. Personally, my favorite vault, in terms of story telling. Dwellers weren't told, initially, that the price of living in the vault was an annual (?) sacrifice of a dweller. In their anger, the dwellers forced the original overseer to be the sacrificial lamb. Since then, the elected vault overseer is sacrificed at the end of his or her term, leading to a strange election where people do not want to be chosen.

Question (major): Who is the fifth survivor (and only survivor of the final suicide)?!

The crazy vault: It's a bit unclear, but it is possible many of the vault's original dwellers were people with mental disorders, including paranoid schizophrenia. The vault dwellers were then divided into a Red group and a Blue group. This, coupled with the sulfur in the air vents and potential subliminal messaging, led to an increased sense of paranoia between the groups.

Question (somewhat major): What happened to the original inhabitants? It is currently occupied by Powder Gangers, who apparently found it empty.

Vault 21: The casino vault. It's inhabitants were compulsive gamblers and everything was resolved via gambling. Mr. House discovered the vault, and won via a bet, the opening of the vault. He then stripped it of parts and filled all but the top level with concrete.

Question (minor): How did Mr. House find the Vault?

The jungle vault. This vault was an experimental vault, looking at horticultural, botanical and agricultural technologies. At some point, spores and chemicals were introduced to the insect and plant populations, and that led to mutations in both, as well as mutations in the dwellers themselves.

Question (minor): Did anybody make it out of the vault? The door seems to be ripped open by the plants, rather than opened manually? Or was it opened to the mercs who were sent by the NCR?

The ghoul vault/the rad vault. The vault had a purposefully large armory. The Boomers originated here, as they left following a disagreement regarding the utilization of the vault's armory. Most notably the explosives stored within. Afterwards, the overseer attempted to 'fix' the overpopulation. That led to a riot/battle, which in turn damaged the vault reactor, which in turn killed or goulified the inhabitants.

Question (minor): Why are there so many barrels of radioactive waste everywhere outside the vault? Find it hard to believe a Vault would be stuck in an area with lots of nuclear waste.

The Fiends probably tossed the bodies out. It wouldn't surprise me if some escaped but I don't believe you encounter any evidence of this.

I don't believe that's known.

Given his connections and technology I imagine Mr. House knew about the vault all along - perhaps even before construction began.

The plants must have burst out of the vault otherwise no one would have known about their growth.

That's a good question. It's possible the waste was put there later since the area was already radioactive but that seems strange - where did it come from and who had the impetus to move it? I imagine there isn't actually an explanation for why the devs put that there.

It's a hop-skip-jump from Lucky 38. Some suspect that House was a resident at 21, but I don't see that as feasible. I think he was always in 38 as his own vault. He could have known it otherwise as he was practically Enclave.