Isolation
Every room is staffed only by a single person. No dweller can interact with other dwellers. This of course would make expanding difficult, but if you’d like you can hold the occasional reproduction night. Alternatively, if you REALLY want to have a bleak vault, you can simply rely on radio broadcasters asking people to come to the vault. Sometimes, the call will be answered—but the radio host will have no idea, because you won’t allow them to meet anyone else in the vault.