In the Fallout universe, Vault-Tec built vaults to protect human life from the atomic bombs. They were supposed to provide safe underground shelter for many years until the surface was safe to be inhabited. The Vault-Tec Corporation actually used many of these vaults to perform sinister experiments on the inhabitants.
Here's the lore for some Fallout vaults I found to be the most interesting in the series:
Vault 11 was an experiment where inhabitants were informed they must sacrifice a fellow survivor each year and If they didn't, everyone inside the vault would be killed. This was actually not true, if the residents refused to kill someone a message would play telling them they are shining examples to humanity and that no one has to die. The vault door would have also been unlocked, allowing them some freedom to come and go with the overseer's permission. By the time they refused to stop sacrificing each other there were only 5 of them left.
The vault dwellers decided that each year they would sacrifice the Overseer. So each year they voted into the Overseer role whoever they wanted to sacrifice. The inhabitants used propaganda to influence other voters decisions on who to sacrifice next.
Vault 22 was designed to be a green vault. All vault dwellers were dedicated to one goal, sustaining the vault population with plants grown within the vault. A fungus was designed for pest control but the fungus wasn't limited to pests, it could also infect human bodies. The fungal spores eventually spread through the vault, infecting the population. The spores would lead to organ failure and death, but eventually it would spread enough that it could take control over the deceased person's body turning them into an aggressive carrier of the fungal spore whose sole purpose is to spread the spores.
Vault-Tec built Vault 75 as a haven for the children of Malden Middle School in case of a nuclear event. Children and their families entered the vault when the bombs fell during the Great War. They separated children from their families and escorted the youngsters into the atrium. The adults were told they were receiving "orientation" but were all executed by security.
This execution was in line with the true purpose of Vault 75 to enhance the gene pool of its residents in order to create stronger and more intelligent subjects. The children were tested both physically and mentally, with anyone that was deemed useless being executed before the age of 18.
Vault-Tec designed Vault 92 to hold 245 occupants for a 100 years. The worlds best musicians were selected for inclusion and the Vault was equipped with an ample supply of sound equipment, instruments and recording devices to protect the musical legacy of the United States.
This wasn't actually the true purpose of the Vault. Vault-Tec wanted to see if white noise generators could be used to implant subliminal messages into the vault dwellers. They wanted to use white noise to create super soldiers that could be controlled through hypnosis.
Eventually the experiment got out of hand, subjects were having outbursts of rage. More than half of the vault dwellers were uncontrollable and slaughtered the rest of their fellow residents.
Vault-Tec created Vault 95 as a rehabilitation facility for drug addicts. The vault would give addicts a place for rehabilitation and a shelter from the nuclear fallout.
The Vault 95 terminal entries show that the rehab program was a success, and the residents overcame their addictions. After many years, a planted Vault-tec agent unearthed a pre-placed stash of chems and alcohol to test the social and psychological reactions of the community. All the vault's inhabitants either died of overdose or in-fighting which left the Vault abandoned until Gunners later used it as a place of refuge.
Vault 108 was created to house 475 dwellers for 38 years. Vault-Tec wanted to use it to study conflict for leadership and power. The vault overseer was known to have a form of cancer that would kill him within 40 months. Positions of authority within the vault were left unassigned, all power was in the hands of the overseer. The main power supply for the vault was designed to malfunction after 20 years, 18 years before the vault door would open.
The study never got that far. The vault dwellers undertook a cloning experiment. They created 54 clones of one inhabitant called Gary. Imperfections in the cloning process caused the clones to become insane and violent, specifically targeting none-clones and wiping out most inhabitants.
This was one of the last vaults to be built. Vault-Tec wanted to suspend 85 occupants in a virtual reality world for the duration of the vault's experiment.
Unknown to the inhabitants, the creator of the virtual reality kit actually had control over the reality pods, giving them no escape from the virtual reality world. He became bored with the various simulated worlds so killed each one of them in virtual reality, wiped their memory and then resurrected them within the program so he could do it all over again.
Vault-Tec built Vault 114 as a place to put Boston's upper class and their families. The upper class that were chosen consisted of local and state government, local luminaries and business people. Future members of the Vault were made well aware they would cohabit with other citizens of similar wealth and influence.
Vault-Tec oversold the Vault, many of the upper-class residents had to live and sleep in single room apartments, share dining and bathing quarters, and amenities were minimal.
Vault 114 staff were interviewing individuals for the Overseer position, specifically trying to find someone with no government or leadership experience and who was strongly anti-authority. "Soup Can Harry" was the man given the job. Soup Can was presumably homeless. He once ate Abraxo cleaner as he states "I seen the back of them Abraxo boxes. 'Not for consumption?'? Don't you tell me what to do, I'll eat what I want!"
Vault 114 personnel were strictly instructed not to undermine Soup Can's authority even if it caused discomfort, embarrassment or harm to the residents. The point was to take away the luxury of the residents and give them an incompetent leader and monitor their reactions to stressful situations.