I also found Vault 34 to be a rather scary experience. I wondered afterwards why I found this particular place so tense (other popular "scary" places, like Dunwich in Fallout 3 did nothing for me). And I came to the realization that it must be the blanket of radiation in the area that just sets me on the edge. I can't remember if it ever goes over 1 rad per sec, but even so, it just gives all the tension in the world to the area and makes it feel much more hostile.
I felt similarily (though less so) about creeping through Searchlight.
Ugh that is exactly what made me despise the place so much. If it wasn't for the two wonderful guns:
Spoiler
The All-American and the Pulse Gun
waiting as a reward at the end I would personally never go down there :cryvaultboy:
The combination of constant radiation and the snarls+padded footsteps of the ghouls gives me the heebie-jeebies. Going through Dunwich without my companions was horrible but going through Vault 34 without companions was MUCH worse. Also I think the reason why most people don't find searchlight that frightening is because you always have the option to just fast-travel out. You don't have that option in Vault 34. You are totally underground, walking tight and dark corridors infested with snarling mindless zombie-like ghouls, while you have a timer ticking at the top left of your screen saying you only got a certain amount of time before you run out of rad-x and radaway and you die. If the ghouls don't get you the damn radiation will.
The only thing that could have made Vault 34 worse would be the reintroduction of Fallout 3 Ghoul Reavers. The damn things are nearly impossible to kill at later levels even with Fawkes and his Gattling laser.