Located in downtown Las Vegas, Nevada, Vault 21 was a unique Vault experiment: everyone in the vault was equal, it was created with a perfectly symmetrical layout, and the dwellers settled their differences via games of chance. Reliance on luck to solve problems created what might be considered the perfect realization of anarchy: a society in which everyone is equal and no one has an advantage over the others. Vault-Tec ensured that every person admitted to the vault when it first sealed was a compulsive gambler, and were unable to decline when Vault-Tec personnel running the vault informed them that this was how the vault was to be run. The original dwellers then passed on their lifestyle to their children and so on.
However, as with all utopias, the peace was not lasting. Sometime between 2271 and 2281, Robert House learned of Vault 21's location. Resisting attempts to be assimilated into his empire, the inhabitants agreed on gambling to settle the issue. The game they played to settle the deal was blackjack and almost the entire vault was in attendance. In a risky bet, the supporters of House won against the opposition, and Vault 21's interior was stripped of its useful electronics and equipment and partially filled in with concrete, forcing the dwellers to resettle in the wasteland and on the Strip. At Sarah Weintraub's and Michael Angelo's (a.k.a. Sheldon Weintraub) insistence, House left the topmost level intact and allowed the Weintraub's to establish a hotel there, which became another source of income for his fledgling empire.
Vault 21 is one of the extremely rare experimental vaults which successfully protected its inhabitants while fulfilling the parameters of its experiment. The experiments in most other Vault-Tec vaults ended up killing or permanently altering their inhabitants. Although Vault 3 was a control vault, intended to recolonize the surface after an extended period of time, when its door was opened the Fiends slaughtered its residents and took over.
The vault's main cog door has been transformed into a sign advertising the vault/casino. "Vault 21" is emblazoned on it in neon lights. The vault entrance is an above ground building which contains the Vault 21 gift shop. It is run by one of the only two native residents to have remained, Sarah. Stairs lead from the gift shop down into the vault.
The vault has been converted into a casino and hotel. There is a central atrium with gambling tables, a hallway with living quarters, including Sarah's room and a guest room that the player can rent. Another door leads to the cafeteria/diner. The lower levels were filled with concrete by Mr. House and are inaccessible except for the maintenance level, which is accessible from the basement of The Tops, where the Chairmen appear to have tried to break in. This level consists of a long hallway with a few small rooms branching off of it. The elevator in Benny's suite leading to it is locked unless he is attacked in the Tops, and the tunnel ends in another locked door.
The Tops houses an elevator that goes down to the basement and connects via a blown open wall to the service areas of Vault 21. This elevator is normally locked and unable to be picked, but is automatically unlocked if Benny is given the chance to flee when confronting him at The Tops.
There is a locked door at the end of a hallway in the vault that can be unlocked using console commands. An invisible part of the Vault will appear if you walk a little forward. There are no lights and this part ends-up with two staircases leading to nowhere.
Although the theme of this vault revolves around gambling, and there are gaming tables around, it is not possible to gamble in this location.
No doors in this vault feature a gold stripe and the number 21, but instead are all marked "No Access". The same is true for Vault 3.
pcps3xbox360 If you take the tour with Sarah Weintraub, your companions may disappear and not reappear when you exit the Vault.[verified]
pcxbox360ps3 Sarah may not recognize Vault 21 jumpsuits as vault gear, but will trade for them normally at the usual price.[verified]
xbox360 Sometimes when you take the tour with Sarah Weintraub, she will walk with you out of the gift shop and then stay still above the stairs leading to The hotel part of Vault 21 and will be unable to talk to.[verified]