Vault 15

Vault 15 was one of the Vaults built by Vault-Tec. Like the rest of the vaults, it was part of the Enclave's grand social experiment. It was supposed to stay closed for fifty years, was overcrowded, and contained occupants of extremely diverse ideologies and cultures. This situation led to a great schism, during which four groups ultimately left the vault to brave the wastes outside. Three of these groups became roving gangs of raiders. . . the Jackals, the Vipers, and the Khans. The fourth group settled down a few miles west of the vault and founded Shady Sands village.

While the three gangs harassed the village and each other over the years, Vault 15 itself gradually fell into disrepair. By the time the Vault Dweller came across it in his search for a replacement water chip for Vault 13, it had undergone complete power failure, several cave-ins, water damage, and other indignations.

Many, many decades later, after the residents of Shady Sands had formed the New California Republic, squatters moved into the area directly over the vault and set up a small shanty town. Shortly after, they allowed a man named Darion and his men move into the vault itself on the condition that they repair the vault's systems in order to help provide the squatters with food, water, and other amenities. At first, this setup was advantegous enough that the squatters rejected all efforts by the NCR to annex them or even gain simple access to the vault.

Then, the Chosen One came along. President Tandi hired the Chosen to infiltrate Vault 15 - or at least get the squatters to allow NCR access - to retrieve computer parts and other equipment vital to NCR's continued expansion. While accomplishing this quest, he (or she) discovered the truth behind Darion's operation. . . he and his men were actually a band of raiders calling themselves the New Khans, using the vault as a base of operations towards Darion's goal of revenge against Tandi and everything the Vault Dweller stood for. Also, the food and water that he had been providing the squatters with didn't come from the vault's systems after all, but the spoils from raids the New Khans had been running along the wastes between NCR and Vault City.

After the ironic death of Darion, killed by the decendant of the man he had sworn vengance against nearly eighty years earlier, the squatters accepted NCR assistance and became a part of the fastest growing republic in the ancient Americas!

Quotes
Vault 15 - "Unfortunately, we were crowded and life was very bad. There was a schism, and many people left, taking with them the best equipment.  Still, some of us tried to stay in the Vault.  But then we were attacked.  I was hurt, and I ended up here.  Now I try to help people..." - Katrina of Shady Sands

Appearances in games
Vault 15 appears in Fallout and Fallout 2.

Inconsistency
The layout of Vault 15 is vastly different in the two games. This is possibly due to the excavation and remodeling by Darion and his crew, but it's highly unlikely that simple raiders would be capable of changes on such a massive scale.