Vault [x]
A vault with narrow corridors and poor lighting and only one place where the inhabitants can get food. Everyone start in parts of the vault that are far from the storeroom. This place is shown on their Pip-boys. Only thing is that the food inside contains chemicals that wipes the vault dwellers' memories, so that every time they've fed, they can't remember anything, go asleep, are transported to the initial location and have to start again with a clean memory.
Since they don't have memories from earlier, they don't know who they or anyone else are.
To make things more complicated, they've all gotten their tongues cut out and some facial surgery to make them look like hell -- so when they meet other people in the dark, they think the others are monsters, can't communicate with them, and will either run or fight. In addition, the vault pumps out a gas that makes everyone tense and paranoid. Yeah, it's not a nice vault.
There might also be one guy who is immune to the memory-wiping effect of the food, but he also falls asleep when he eats it, and is transported back to his "spawn point". He hasn't got a tongue either, but he's got a chainsaw (which helps).
Vault [y]
5000 people. Standard food and equipment supply. In the central room of the vault, there's a giant pit with a http://orangemonk.files.wordpress.com/2010/03/sarlacc2.jpeg in it. If the Sarlacc is not fed once a week, the vault computers will on the first day overdue initiate earthquakes, then overcharge electrical circuits, then cut food supply and finally release hungry beasts to devour the vault dwellers.
The point here is that the vault dwellers have to sacrifice one of their own to the sarlacc in order to survive. The general Vault-Tec hypothesis is that a religious system centered around the sarlacc will soon evolve, with a group of the vault dwellers taking control.