Vault 21

 is a Vault-Tec Vault, that was once hidden under Las Vegas, Nevada. It has since been converted into a hotel/casino.

Background
Located in New Vegas, Nevada, its purpose in the Vault Experiment was gambling, reinforced by having only compulsive gamblers admitted as Vault residents, and all conflicts within the Vault were solved through gambling. Mitchell and Sarah come from here. At some point, Mr. House decided he wanted to own Vault 21, so he bet for it. He won and filled most of the vault with concrete, initially wanting to fill it all. His motivation for filling the vault with concrete is unknown but could be to either conceal whatever went on in there (as he is assumed to be the Overseer of that vault) or simply to strengthen the foundation of the huge casinos he built on top. Sarah Weintraub convinced Mr. House to leave a part of the vault and it was and converted it into a hotel/casino.

An interesting particularity of Vault 21 is that it seems most of its inhabitants are agoraphobic (Sarah Weintraub and Michael Angelo) and dislike the idea of going outside. This could be because, psychologically, it is hard for them to cope with the outside world where not every decision is left to chance. The same is true about Mr. House's thinking rigidity, as he is obsessed with controlling those odds to a tee, hence his very complex plan and his constant forcing of the player to follow the plans very precisely. An exception from this assumption would be Mitchell but he doesn't seem to leave his house either (doesn't go to fight the Powder Gangers and the Courier is brought to him by Victor). Another evidence of agoraphobia in Mr. House would be the fact that despite all his advanced technology and almost unlimited resources he did not create anything to help him leave the Lucky 38 and his plans for the world are mostly centered around the development of New Vegas.

Layout
The Vault has been converted into another casino and hotel. The Vault is actually advertised with a big sign, unlike all the rest. It's possible the Overseer is Mr. House. It is a large maze of tunnels that is built around a central atrium. This Vault does not appear to have an Overseer's office, and its lower levels are filled with concrete by Mr. House, because "maybe he doesn't want anybody sneaking around down there" (Mentioned by Sarah). The Overseer's office may be in those concrete filled levels, buried by Mr. House in order to conceal his past. It is accessed by a small building which contains the Vault 21 gift shop, run by the only native resident to have remained, Sarah, with stairs down into the Vault in a back room. The Vault's main cog door was used as the sign for the vault with Vault 21 neon lights put over it.

Appearances
Vault 21 only appears in Fallout: New Vegas.

Behind the scenes
Vault 21 is a reference to the card game "Blackjack", where a player has to have the highest sum of card numbers without going over 21.

Bugs

 * If you take the tour with Sarah Weintraub, your companions may disappear and not reappear when you exit the Vault.

Krypta 21 Убежище 21