Find the Underground Vault

Find the Secret Vault is a main quest in Fallout: Brotherhood of Steel. It is started in the Los holding pens.

Walkthrough
After slaying Blake, get the key to free Rhombus. The paladin knows the way to get into the secret Vault-Tec Corporate Vault, but has to lead you to the key that opens it. Follow Rhombus as he runs down the alleys. Finally, Rhombus reaches the secret stash, takes it, but gets blown up by a kamikaze ghoul, injuring the knight. Rhombus hands over the key and a Brotherhood insignia, which allows entering the warehouse. Backtrack to the bridge and talk to Giese, who admits to having more information about the vault, where he used to live as a technician. He shares his knowledge on how to enter the warehouse.

Return to the docks via the shortcut. Start moving southeast to reach the Vault-Tec warehouse, marked by the shiny letters "VT" on the side. Use the alley to the east to reach a small courtyard with a few puddles of goo and the first encountered super mutants of the game. One of them has the VT warehouse key so kill them to get it and enter the warehouse. Progress down a set of stairs in the northwest corner of the warehouse, which lead to a room with six turrets, the last area before the Vault.