Just a FYI that this is an unmissable achievement that will unlock just after starting Chapter 2: Sanctum.
When the chapter starts, after the cutscene, and if you are playing as Keegan, you gain control of your character, walk forwards off the platform and head into the small control room on the left. Use the control panel to activate the giant vault door in front of you. Walk towards the door as it starts to open and once the door has finished its opening animation, and you have access to Sanctum, "POP".
If you are playing as Mac or Lahni, then computer controlled Keegan will automatically go to the control room and open the door, and as such the achievement will essentially pop automatically, 30 seconds after starting the chapter.