The Fallout universe is a post-apocalyptic nightmare world of radioactive mutants, monstrous abominations, and malfunctioning killer robots, all out to murder and maim random wastelanders in various horrific ways. Yet the games never forget to remind the player that often the worst monsters are human beings, and few titles in the series do this better than in New Vegas.
As in previous games, while the infamous vaults by Vault-Tec were ostensibly meant to be long-lasting fallout shelters for evacuees, only a select few control vaults actually worked as advertised, with the vast majority acting as sick social experiments by the company to test the limits of human nature. New Vegas features some of the worst Vaults in this regard, and the player might be terrified to know the depths of human nature.
Vault 22
Designed as a "green" vault, the inhabitants of this vault consisted of scientists dedicated to creating crops that could grow in the wasteland and combat global hunger. Their experiments actually went very far, as the player finds that the vault is filled with living plants lush with a healthy green color.
However, this vault's creators clearly never intended to show any consideration as to what the scientists could experiment with, tampering with an uncontrollable fungus that ended up vegetizing all of the staff. All that's left of the vault's residents are their zombified plant corpses and mutated venus flytraps, all hostile to anyone or anything approaching them.
Vault 19
Unique in its assignment of two vault overseers, Vault 19 attempted a sick social experiment involving segregating the entire vault populace into two sectors: red and blue. The goal of this was to induce paranoia and prejudice for the opposing faction without the use of chemicals or outside factors.
Journal entries in vault computers, as well as observational clues, reveal the vault's inhabitants becoming more and more afraid of each other, blaming the opposing sector for various technological and maintenance malfunctions. Perhaps the most terrifying aspect of this vault is that what happened to the residents is unknown. By the time the player finds Vault 19, it's already overtaken by the Powder Gangers, but it's reasonable to assume the red and blue sectors ended up killing each other in a civil war.
Vault 34
Simultaneously satirically funny and psychotically disturbing, Vault 34 is the former home of the highly xenophobic and gun-crazy Boomer faction. The vault was completely overstocked with weapons, ammunition, and recreational tools like a swimming pool, all at the cost of living space, which ended up creating an obsessive gun culture and severe overpopulation.
The faction that would become the Boomers managed to leave the vault and settle elsewhere, but the remaining dwellers were forced inside by the overseer amidst protests about their right to bear arms, sealing the exit behind him. A violent revolt occurred in a failed attempt to overthrow the overseer, which resulted in stray bullets causing a damning radiation leak. The player will find the residents inside have already transformed into feral ghouls.
Vault 3
This vault contains probably the most tragic history amongst all the vaults in the series, as Vault 3 turns out to be one of the few control vaults that worked exactly as intended. The only significant issue was a water leak that caused the inhabitants in the vault to send someone to the outside world in order to receive help.
This proved to be the end of Vault 3's history, as opening the vault door led to the monstrous raiders known as the Fiends to invade and pillage the dwelling. Perpetrated by the vicious Motor Runner, the residents of Vault 3 were all unceremoniously slain, enslaved, or worse, providing a horrifyingly realistic portrayal of what enacting blind idealism and obliviousness in a post-apocalyptic setting would entail.
Vault 11
Perhaps the most heinous social experiment on the list, the inhabitants of Vault 11 were notified by a supercomputer that one of their own must be sacrificed or else the entire population will be killed. This created a fearful society that engaged in an annual election for the next overseer, who would be the one to be sacrificed. Eventually, a resentful overseer's override caused the computer to randomly select a sacrifice instead, creating a mass panic that led to most of the vault's dwellers to kill each other.
The remaining five residents, tired of the atrocities, refused to take another life at the risk of all dying together, only to discover that this is what the computer had wanted: by refusing to kill and showing themselves as "shining examples of humanity," the computer would have let the entire vault population live decades ago, making the years of blood, trauma, and death completely and utterly pointless. The player finds that four of these last survivors committed suicide out of guilt, while the fifth left to parts unknown.