Vault 111 is one of the 122 fallout shelters constructed in preparation for total atomic annihilation. This shelter, like all Vaults, was built by Vault-Tec and designed for humanoid occupants.
The Vault is reached via large elevator which descends to show the Vault Door itself. This door leads into the main reception area, which is linked to a service tunnel and a long corridor connecting two Cryo-Pod Storage rooms and another link to the maintenance areas, staff quarters, and Overseer's quarters. There is also a water purifier beneath the bridge connecting to the Vault Door.
Vault 111, like most fallout shelters, was secretly an experiment. The 24 occupants of the Vault were members of the "Nightfall Project," which were unaware of the fact that they would be frozen to preserve them in case of the Midnight Coalition's destruction, so that the cause may be revived later on. However, Vault-Tec also used this Vault as on opportunity to test Cryo-Stasis on a number of subjects to see if they could survive.
Of the expected 24 occupants, only 17 arrived. The other members had either been killed prior to the Last Lights or did not make it to the shelter before the experiment began. Connor Pemberton (also known as Apex Firebug) was one of the members of the Nightfall Project, but was caught outside during the nukes with another member. However the two managed to find the Vault and get inside while Connor's Titan, a Scorch named Blaze, remained outside to guard the area despite the fact that his chassis would certainly fit on the lift.
The subjects, having waited half an hour for other potential subjects to arrive, were then told to put on Vault 111 Jumpsuits and enter the Cryo-Pods, which they were told were Decontamination Pods. The men and women got into the pods and were frozen solid in seconds.
As time went on, Vault-Tec staff inside Vault 111 began to question the experiment. As the occupants were maintaining vitals, it was predicted that they would outlast the staff by years or even decades. As food inside the Vault grew scarce, the staff started to turn on one-another and a few escaped to the surface only to be "cooked" by the intense radiation very quickly and die. Eventually, the entire research team was wiped out.
With nobody to watch over them and Blaze out of energy due to 200 years of use, the Vault was targeted by the Institute as a way to gain pure British DNA in an attempt to revive the species, as well as retrieve Connor as a request by the Synth counterpart of Director Topaz. However no subjects would come in quietly and some were driven mad by the freezing process, with a few subjects having died due to improper thawing. Connor managed to fight off his attackers and was the only survivor of the Vault, setting off to find his wife and daughter with no knowledge that 200 years had passed.
Vault 111 went untouched for a few months until Scootaloo and Sevens came across it and made it their home. While Sevens would be at home on the lift (his chassis couldn't fit deeper into the Vault) Scootaloo blocked up the tunnels further into the Vault and made her home in the main area, with her bed located in a blocked-off maintenance tunnel. She decorated her abode with salvaged items such as traffic lights and trophies taken from slain foes and animals. A memorial for her fallen friends (Sweetie Belle and Apple Bloom) was constructed in another blocked tunnel using metal scraps and relics from the two including the unique weapons "Sweetie's Belle" and "Blooming Apple." She used the Vault as both a living space when she was not pursuing a quest and a place to store her more valuable items, including unique weapons and armours she did not want to carry with her at all times. Aside from Sevens in the lift, Scootaloo had the entire Vault to herself and defended it from many foes including the Institute, Enclave and New Lunar Order.
The Vault continued to be Scootaloo and Sevens' home for years upon years, and after Scootaloo's death it was sealed off indefinitely at Sevens' will. He asked Blaze and Connor to melt the lift's door in place and bury it as to keep it's secrets safe and secure, as well as leave Scootaloo's body undisturbed. Sevens entered permanent shutdown after the door was sealed, and the Vault was never entered again.