I have this problem too

It would seem its caused by having pursued more than one of the technologies that Veronica suggested. I currently have the pulse gun and the rangefinder gun too in my inv and Elder Hardin and Veronica won't converse to return the gun. Or rather they are talking cause I cant select either of them at the time, then they stop and I can only choose 3 bits of dialog from the elder, and the normal stuff from Veronica.
Obv this is a pain in the [censored] as I wanna get this long ass quest finished. PC Users can skip by this by using the console, but 360 and PS3 players are screwed, unless someone knows a way?
I'm gonna try and have a fiddle with the computer in GIbsons Shack tonight and try dropping/picking up the weapons one by one and see if one of them triggers the speech.
Otherwise its clear cache time, then failing that, loading a save that was 2 hrs prior
Ty Bethesda

I trusted you