Vault doors appear in several missions in City of Villains. Generally there is loot on the other side of the vault door. To get to the loot, the vault door must first be destroyed.
Typically, when a villain comes in close proximity to a vault door, several waves of security are summoned to the villain's location. After destroying a vault door, several more waves of security are summoned to the villain's location. This security is able to see cloaked or invisible villains, thus making powers such as a Stalker's Assassin's Strike very difficult.
The vault door is very tough to break through, and when it is destroyed, it appears to explode. However, when it explodes, there is no damage incurred on any villains in the proximity. There is experience and infamy awarded to destroying a vault door.