By the year 2242, Vault 15 - or rather, the area above it - had become inhabited by a group of squatters that had previously been wandering the wasteland. They set up a small shanty town and shortly thereafter became engaged in a battle of wills against the New California Republic. The NCR wanted access to the vault their ancestors had come from while the squatters simply wanted the NCR to shove off and leave them alone.
Just when things seemed bleak for the squatters and it seemed they would have to give in to the NCR's demands, a man named Darion appeared and offered to help them repair the vault so they could become completely self-sufficient and resist the NCR's advances indefinitely. The squatters readily agreed to this plan and allowed Darion and his men entrance to the vault.
Darion appeared to be holding up his part of the bargin for quite some time. Food, water, and medical supplies began to pour forth from the vault in small doses, proof that he and his men were making good on their promise to fix the vault's systems. Unfortunately, things soon began to take a dark turn. From time to time, members of the squatter community would disappear, and the men that Darion was bringing into the vault appeared to be of a sort that the squatters would not normally want to associate with.
For one squatter, one of these dark turns was one such turn too many. When the Chosen One came to the squat, Rebecca approached them about her missing daughter, Chrissy. She voiced her suspicions about the men now populated the vault and asked the Chosen to find Chrissy, a quest that ended up aiding them with their mission of finding a way into the vault.
After the revelation of Darion as the leader of the New Khans, the squatters accepted the assistance of the NCR in return for Vault access.
While the squatters aren't exactly xenophobic, they greatly value their privacy. They prefer to be self-sufficient, but seem to have trouble attaining that goal completely. In the end, what they want more than anything is to be left out of the strife and politics that plague places like NCR and Vault City. The squatters are led by Zeke, a capable but harried man.
The Vault 15 Squatters appear only in Fallout 2.