It takes two keys to open the vault. They have to be turned simultaneously. The other one is in a locker at the security station.
8 seconds sound clip from the Terminator 2: Judgment Day movie soundboard.
You can hear this line at 01:47:48 in the Blu-ray version of the movie.
Quote context
[...]
- Friends from out of town. I just thought I'd take 'em upstairs and show 'em around.
- You know the rules concerning visitors in the lab. I need written authorization...
- Come on, man! You can't leave the desk like that!
- Gibbons!
[...]