Allright, let's see if I can get it straight now...
So the commercial airways crossing Laos and Vietnam airspace would've been in use throughout the war? Certainly some areas where large scale operations took place would've been off limits to airliners during those years?
Indeed there was of course a whole lot of flying and dumping of hardware going on as Speedpig points out, so the chances of an encounter wouldn't be as small as I originally thought.
Seeing as Laos and Vietnam airspace is quite close together (not that easy to cross one without getting close to the other) this means that the encounter described is very well possible. Factor in the fact that even though there was no official war in Laos, there were certainly a lot of military around the place and that only adds to the possibilities.
Anybody with some more additions to this?