Vault 74

Vault 74 is one of the Vault-Tec Vaults located in the Capital Wasteland. Not much is known about its fate except that it was overrun by raiders. It's small, consisting of just an atrium, a clinic, and an overseer's office. There are living quarters but the only way in is blocked. The state of disrepair in the Vault is most likely due to the fact that everyone in Vault 74 perished, and that the Raiders had raided and looted Vault 74.

Related quests
The Vault comes with a simple quest called "Exploring Vault 74". You start the quest by simply entering the vault and you complete the quest by taking the Vault 74 Overseer's log:

Purpose
While there is no given purpose of the vault directly given to the player (either via terminals or word of mouth), it is somewhat implied through the Overseer's terminal that the Vault was provided with less power and energy than advertised and intended, resulting in eventual failure.

Appearances
Vault 74 is a Vault that the Bethesda Softworks developers made for the official G.E.C.K. tutorials and it does not appear nor is mentioned anywhere else. Given that, the location is likely non-canon. This means that This Vault can't be found in any console versions of Fallout 3, only PC. This Vault also appears in the game files of Fallout: New Vegas although is only accessible through the use of the console command 'coc vault74a'.

Related
Vault 74 Overseer