After slaying Blake, get the key to free Rhombus. The paladin knows the way to get into the Secret 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 gives you the key and a Brotherhood insignia, then charges you with entering the vault. Backtrack your way to the bridge and talk to Giese, who admits he has more information about the vault, where he used to live as a technician. He will tell you 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.
Kill the ghouls and mutants as you go through the sections while making sure to get the good armor and weapons for the road ahead (see warehouse map). The last area before the Vault is a room with 6 working turrets. Destroy them and enter the vault.