Vault 84 has been closed for almost its entirety, with few outsiders being welcomed inside. They had a supply line and trade with Vault 109 before that vault's destruction. Each year, there is a vote held in the vault to exile who the citizens deem the most dangerous. Whoever obtains the most votes may never return to the vault. Overseer Hayes lets you wander into the vault after suspicion has fallen on her, as she believes letting in some "fresh blood" will help. After reading her entry log about the voting system, she will be exiled during the vote once exposed.
↑"Welcome to Vault 84, I'm Overseer Hayes, the leader of this vault. We haven't let in outsider in over 100 years" - Card 71
↑"Log: The concept of 'the traitor' is a pre-war fabrication perpetuated for psychological study. I thought this tradition harmless until this year when suspicious fell on me. Perhaps some fresh blood would help" - Card 74