Vault 13 is occupied by a merry band of sentient Deathclaws. They are sentient and they are civilized, so no need to draw your weapons. Talk to the one closest to the vault door, ask how come he speaks so well, ask about the GECK, ask to be let to look for it, and offer to help in exchange. (You did bring the voice recognition module along, didn't you?)
Go down to level 3, click on the terminal and select the choice to
open it. Use the voice recognition module on it.
While you're on this floor, you can talk to Goris and even take him in
your group, if you have places left. Bad part is that, just like
Marcus, he can't use armour. Worse yet, he can't use weapons, either.
Good part is that he gets even better hp than Marcus once he levels up
a bit, and he can dish out awesome unarmed damage. When he's maxxed
out, I think he gets like 41 unarmed damage, and 4 attacks per turn.
And I highly suspect that his claw adds to that. I.e., if you need
someone to dish out melee damage fast, Goris is the man... erm... the
deathclaw. Of course, that'll mean he's always in the way of your
shot, just like anyone else using melee weapons.
A problem you will have sooner or later is that when you arive in a
location, Gorris will leave, saying his pack is in trouble. Usually
it happened at the military base, but last time it happened in San
Francisco. You can go pick him back from Vault 13, after you watch on
the security cameras the Enclave troopers slaughtering Grunthar.
However, the endgame animation will say that YOU slaughtered the
sentient deathclaws. WTH? Oh well...
You should also thoroughly loot the base. On the third floor, be sure
not to miss the NavCom parts, which you'll later need to repair the
Valdez tanker and go to the Enclave base for the final showdown. You
can also get another combat armour. Ok, I know you have tons of
combat armours already, but you can use up to _six_ of them for parts
for the combat implants, if you decide to do those. In another locker
there's the "holy" GECK you were sent to retrieve. Taking it is worth
4000xp. If you don't take it now, Grunthar will give it to you
anyway, when you tell him you fixed their computer. (And you'll get
the xp, too.)
You can also talk to their Mother Deathclaw, to find out more about
Matt, the prisoner on level 2. I.e., he's not kept there just for the
fun of it, but because he tried to plant a bomb and destroy their
eggs. And they're quite nice about it, if you ask me. I.e., don't
try to rescue the moron. If you try, you'll end up fighting (and
killing) everyone on level 1, including the doctor, and Matt will run
away after the slaughter anyway.
Be sure to talk to Grunthar on the way out, regardless of whether
you've taken the GECK already or not. That's when you get the xp for
fixing the computer.
OK, since now you have the GECK, let's go back to Arroyo...