Vault 87 is one of many vaults in Fallout 3.
Vault 87 is located at the west side of the Capital Wasteland. Its main entrance is highly irradiated and will kill the player in a matter of seconds. The only way to access the vault is via Little Lamplight.
Like many vaults, Vault 87 was an experimental area to research new things on the human population. The purpose of Vault 87 was to inject humans with the Forced Evolutionary Virus in hope that they would mutate into super mutants which could then be used on the battlefield to fight the Chinese army.
The Vault is believed to be haunted by the ghosts of the failed FEV experiments which can be found in many mutation chambers around the vault. However, this is most likely false, and the vault is simply home to super mutants.