Aperture Science Extended Relaxation Annex

 the Vault  is a large room located in Test Shaft 02 of the Aperture Laboratories, acting as the main goal of the fifth course, Mobility Gels, in the Portal 2 co-op.

It is a large room containing hundreds, possibly thousands of humans in cryo-storage, sealed off probably for use in tests later on. During the Portal 2 co-op, ATLAS and P-body installs four discs, fully GLaDOS, allowing her to see everything within the facility, and showing the location of the Vault to her. She then sends ATLAS and P-body into Test Shaft 02 to recover the humans, as she has concluded that testing with immortal robots is not real science. ATLAS and P-body then travels through a testing course to reach the Vault, and manages to open it using gestures, allowing GLaDOS to begin testing on the humans.

It is possible that the Vault was protected against robots, as a turret says "but I need to protect the humans" as ATLAS and P-body defeats it, and that the door can only be opened if the access camera sees humans, although ATLAS and P-body still manages to open it by acting as humans through gestures.