thefifthhorseman: When going to the bottom level of the vault, you should receive a message in the status box stating that you won't find it there.
Kill the rats, use Science on computers, collect the loot and go back.
You should have the location of Junkyard on your map - that's your next stop.
If you don't, you need to talk with some of the people in Shady Sands.
(a) Junktown is a good next stop, but not necessarily the next place your character will think to go. Some luck and a little wits might even put you near the old Bakersfield ruins much earlier. Still, if the player's that stuck, I do agree that Junktown's the place to go.
(b) Just for the sake of asking, are you sure there are uses for Science on the computers in Vault 15? I've played the game a goodly many times and have never found those rooms useful for
scientific purposes. Other uses, sure, but Science?