A Vault computerClosed from the inside by a reinforced high-security door and from the outside by a massive, gear-shaped four yard thick vault door (which Three Dog claimed "weighs, like, thirteen tons"), it's the only means of entering or leaving the Vault, although secondary entrances or exits may have existed in some Vaults.Vault 87 had two additional entrances reachable through Lamplight Caverns. Security codes are required to both leave and enter the Vault, and they are usually only known to a handful of people within the facility. East Coast Vaults did not seem to require codes, but instead they had a console located on both sides of the entrance, which opened the Vault door via a lever.

The automated narrator of the the Vault-Tec vault demonstration in Washington DC's Museum of Technology states that the doors had a projected 2% failure rate in case of a direct hit by a nuclear missile. So far, the only known vault to have been hit directly (or very nearly) by a nuclear weapon is Vault 87. According to the terminal of Vault 87's overseer, the blast caused the vault's main door to completely and utterly fail, apparently damaging it "beyond repair." Though this could be considered unfortunate and ironic happenstance, it is more likely an indicator that Vault-Tec's failure rates were completely fabricated.

Most Vaults use a Seal-N-Safe Vault Door Model No. 343[8] to secure the airlock, however, some older Vaults (such as Vault 101) use a different, more crude blast door model. Vault 8, the control Vault, had also a second, much larger, blast door built, that secured the entry hallway leading to the entrance to the Vault.

In addition, the Entrance level also houses the Emergency Medical Lab complete with an AutoDoc. A Vault medic was required to be present at the EML 24 hours a day. The lab had the equipment to treat nearly all injuries and illnesses, ranging from simple bruises to irradiation.