When I go to open vault 3 it says the Vault is opened from elsewhere. Where is this?

SPOILER

In my game I took the mission to kill the Fiends at Camp McCarren and killed most of them around the Vault and I have killed the 3 Fiends that I needed for the mission for the NCR. When I first completed my game Camp McCarren is overrun by the Fieds so I want to wipe them out.

Is it becuase of the NCR mission I will never be able to open Valut 3?

Rather than clicking on the door itself, you need to click on its control panel, which should be right next to it.

Thxs. How stupid of me.