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 former 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 as it was and converted it into a hotel/casino.

Another quirk of Vault 21 is that most of its known inhabitants seem to be agoraphobic (e.g. Sarah Weintraub and Michael Angelo) and dislike the idea of going outside. It is not surprising that many Vault dwellers might feel this way, being brought up and living most of their lives in an enclosed and controlled environment. Doc Mitchell might seem an exception, but he doesn't seem to leave his house (to fight the Powder Gangers or to meet the Courier, who is brought to him by Victor). Mr. House is also obsessed with control and tilting the odds in his favor, hence his very complex plan which he requires the Courier to follow precisely. Despite all of his advanced technology and resources, he did not create anything to help him leave the Lucky 38 and his plans 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.
 * In the dining area, a radio may be seen partially stuck in the central wall of this area.
 * You can find a Snowplow in

Krypta 21 Убежище 21