A few months ago I sold Fallout: New Vegas since Dead Money was no where to be seen on PS3 and I already hit level 30 on two characters.
I recently bought it again with Dead Money and Honest Hearts and I've been salivating at the mouth for Old World Blues.
There is just one problem I'm having with my game.
 
I started Return To Sender before I sold New Vegas and when I bought it again the first thing I did was do both DLCs. Well I forgot about the broken quest and now I can just reload an old save.
The glitch is

This thread made my Giant Bomb day. 
 
All of the beautiful spoiler tags... ahh....

Have you checked out the bugs section for the quest at The Vault?

I had the same issue. I can't at all remember how I fixed it. I believe I just talked to a bunch of the dudes that you need to talk to (umm, I  want to call them 'rangers'). There were steps leading up to the mansion from the small camp. And one was on the steps. Then I finally heard the bang. Was stuck here for roughly 2 hours. 
 
Sorry, recalling from incredibly poor memory.

I told Chief Hanlon that i was going to turn him in and swiftly left his office. The door locked behind me and i never waited for a gunshot. I left the building and that's when i noticed that there was no gunshot at all. i wondered around and talked to every ranger in the damn camp. I ran around for about 3 hours before giving up on this achievement. Today (a month later) i came back to this mission because it annoyed the hell out of me that everything was completed but the mission was still on my pip-boy as if i never finished the mission. I have figured out a solution to this. (Chief Hanlon locks himself in the room and wont commit suicide). note - throwing the grenade crap does not work. that only makes the NRC attack you. planting C-4 and crouching so that you are "hidden" so you can detonate it does not work either - that only kills Chief Hanlon and you fail the mission (no achievement) I would try to explain how to fix this but its far too weird to try and explain. I will be posting a video on youtube tomorrow most likely and then ill post the link here...