Descend into the vault through the sewer tunnel in the shack. Inside are caves similar to those outside of Vault 13. Walk through the entrance and you will notice that this Vault has the same room design as Vault 13. Keeping that in mind, check the storage locker on the west wall. Inside are a bunch of flares. Through the next doorway you should be greeted by a mole rat and a few of his fellow cave rats. Shoot, bludgeon or cut them as you see fit. Past the west doorway and on the other side of the glass wall is another locker. It contains a few of the valuable stimpaks and other medical equipment. Go back to the crossroads and go east this time. There is elevator shaft there. Use the rope you should have gotten from Shady Sands or Junktown or the Hub on it. Climb down.

Second floor, this is the living quarters. It is identical to the living quarters in Vault 13 except for the lack of environmental control, caved in tunnels, and infestation of large rodents with attitudes. This is also one of the few places pig rats exist. On the north side of the living quarters, is a caved in tunnel. Near the ruins is a box of 10mm ammo. The lockers in the south rooms contain a suit of leather armour and a coil of rope. If you haven't found a better suit of armour, put it on. Find the elevator shaft and use the rope on it. You will gain access to Vault 15 levels 2 and 3. Make your way to level 3 and head for the south-eastern corner. There you will get some experience and will be informed that you won't get the water chip here. Search the lockers. Inside you will find a Heckler and Koch MP9 submachine gun and some ammo. There should also be some dynamite in one of the lockers. Exit the vault.