The final three Vitrified Doors can be found here. The next day, Kris and Susie return to the Dark World to confirm it was real, where they reunite with Ralsei. Puzzle your way through Aperture's subterranean areas. Enter this door to find a secret hallway that leads to a dock (see Ship Overboard Achievement /Trophy). Gameplay straight from the mind bending, long awaited puzzle shooter game Portal 2. In the corner of this office is a cabinet with a door behind it. You can use a portal to get high above the Control Room and enter a small office. Below one of the test spheres and a giant Aperture logo in its 70s cooper Black Font is a room marked CONTROL ROOM in yellow lettering. At this point, Cave will start talking about testing on hobos and you'll know you are in the right area. Now, play through the level until you get to the 1971-era test facility. Listen to the audio that plays when you hit the buttons in front of each of the sealed doors. Place a portal above the moving wagon to your right and then use the blue repulsion gel to jump on the ledge behind the gel. On one end of this chasm is a platform with three of the Vitrified Doors. You must use the elevator shaft as a gravity boost to get across a chasm.

After flying through the Aperture logo you'll come to an area with a tall elevator. At the beginning of Chapter 6 you'll have to penetrate the 1960s Aperture facility. In Chapter 6: The Fall, there are six sealed metal doors marked VITRIFIED ('Vitrified Doors') that have buttons in front of them that play unique audio clips of Cave speaking.

Portal 2 chapter 6 the fall walkthrough