So I've completed everything through Vault 22 and I'm at the part where I've spoken with what's her face and I run down to the 5th level to ignite the gas. I have the location and know to take refuge in the terminal room you downloaded the information in. I know what to do, just it won't do it. When I was originally standing in the hallway herping & derping around, I accidentally lit the place up after noticing the shimmering effect. I reverted to an earlier save from around the corner and went to hide in the room I'm supposed to, fired... no go. Laser weapons, plasma weapons, grenade launchers, frag grenades, nothing will set it off again. :\

Is this a bug or am I doing something really, really stupidly wrong somehow? (For the record I spoke with the ghoul again and went through all the conversation parts then tried again, and I know I'm in the right location (mostly because I blew myself up the first time like a true [censored].)

I understand your frustration. I used a long fused stick of dynamite to trigger the explosion and it still took me 4 tries to finally trigger the gas. Keep trying and try to throw the explosive projectile as far down the hallway as possible.
Good Luck.

But why could I set it off just shooting down the hall earlier and yet now I can't set it off doing the same thing OR using a remote explosive while hiding in the room? :\

Edit - I can also get the door shut before the explosion goes off as well.