The Vault Door connects the Vault to the Wasteland. It is an entry point for new dwellers, explores and the first line of defense in case of Raider attack.
The Vault Door is a fixed entry point that can only be upgraded to increase its resistance. By upgrading you can prolong the amount of time it takes for raiders to gain entrance. Additionally you can station two guards to defend against Raiders or Deathclaws for when they breach the door. After they breach and are killed the door will repair itself and prevent gases from the wasteland to pour into your vault.