80年后， 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 descendant 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 former continental United States of America.