Vault-Tec Calling! We are pleased to reveal your new home underground. Despite an initially chilly reception, you may find this location to be the safest around, especially as the population has thinned out quite considerably since you were frozen.
Vault 111 was constructed to observe the effects of long-term cryogenic stasis on unsuspecting test subjects, most of whom were residents of the nearby neighborhood of Sanctuary Hills and the town of Concord. Due to the nature of this experiment, the Vault was assigned only a skeleton crew of scientists and guards. The staff were only supplied for 180 days, with directions to evacuate upon receiving an all-clear signal from Vault-Tec. After evacuation of staff, the test subjects would be monitored remotely.
However, as the staff soon discovered, no all-clear signal was forthcoming. As supplies dwindled, the vault's security team led a mutiny against the overseer, planning to open the vault door and escape. The ensuing firefight killed many, and any survivors presumably left the vault. The fate of these men and women is unknown, although it is highly likely that they died soon after leaving. The test subjects, frozen in their pods, remained untouched for over a century.
Vault 111 is a very small facility compared to others. It is made up of only a few rooms and corridors. The largest two rooms house the cryo pods in which the unwitting test subjects reside. The vault also contains sleeping quarters for the staff, a cafeteria, and a reception area. The overseer's office houses an escape tunnel, which collapsed at some point.
The staff populating the Vault was doomed from the beginning: The "local" fallout that would result from an apocalyptic all-out nuclear war would be lethal for between twenty to thirty years, thus rendering much of the surface of the areas affected by nuclear strikes uninhabitable; 180 days is nowhere near enough time for radiation levels to have dropped to "background," or safe, levels, something Vault-Tec likely knew and the overseer was aware of. As mentioned in his terminal, all Vault 111 staff were considered expendable, supposedly (but not actually) excluding the Overseer. The limited supplies ensured that the staff either starved to death in the Vault, or rapidly cooked to death outside of it.
↑Fallout 4 loading screen hints: "Those who reserved spots in Vault 111 had no idea they would be cryogenically frozen. In fact, almost every Vault was actually a secret social experiment conducted by Vault-Tec."