This quest is an unholy mess and somebody should be ashamed. I am working on this from the NRC side, with both the latest patch and Dead Money installed.
What happens to me:
1. I enter the tower. I understand I should be hearing a radio discussion between the traitor and the Legion, but there is nothing no matter how long I wait.
- If I leave tower at that point, I won't be able to get inside the monorail, nor will Colonel Hsu acknowledg anything.
- Thus, my only option is to go up and at that point the traitor attacks me yelling "The trail ends here" or something like that at which point he gets killed.
2. The quest updates with "Go talk to Colonel Hsu".
3. Many people say that I should go directly to the monorail, but this is impossible, if I go it only offers me a trip to the Strip, I still cannot enter the monorail itself. So to Colonel Hsu it is.
4. Hsu sends me to to the monorail, everything goes well and I diffuse the bomb, no issues. Quest updates that the bomb has been diffused. Does _not_ show it on the screen, but when I view the quest log, there has been another update is that I should go back to Colonel Hsu.
5. I go to Hsu and he asks me "What happened?", but gives _absolutely no_ (as in zero, zilch, nada) reply option to tell him anything new. All I want to do is tell him, "Yeah, it's saved, cough up the XP" but no. I get completely stuck at this point, nothing I can do, Hsy doesn't update his speach options so that I could tell him anything about the monoral.
The End.
I've gone through many, many, many discussion threads on this quest online, so many different problems, so many different solutions or workarounds, but not for this. Even after patching this quest still doesn't work properly and someone needs to take a long hard look at what's wrong. Preferably the person who was/is responsbile for this unholy mess to begin with.