I can't help you in finding one, but I carried one in the Army. Every Aviation unit I went to had them and Ruger 4" Service-Six's with round butts. (Yeah, I know that's a Speed-Six in the civilian world, but they were marked "Service-Six" and had round butts).
It wasn't Vietnam era though, it was later in the 80's and 90's. By the time the M9 got adopted, nearly a third of the sidearms in service were wheel guns. The M1911A1's were shrinking in number because they hadn't bought any new ones since WWII. So they made up for numbers in just buying batches of revolvers and issuing those because of all the false starts to replace the M1911A1. We had the revolvers in Aviation, which was the biggest user of sidearms. Some MP units used them for Dog Handlers, because it was easier to shoot one-handed and you needed one hand to control the dog.
Other than that, it was a box stock Model 10 with commercial blue finish. 4" standard (i.e. "tapered") barrel with a round butt. They weren't shot much, but carried a bunch and not cared for well. The GI issue .38 FMJ round wasn't something to inspire confidence either. It wasn't really any different than today's WWB, etc.
We had shoulder holsters, hip holsters and sometimes a holster sewn onto the survival vest. Most of us just wore the shoulder holster and fed it thought the arm hole on the vest, rather than swapping to the vest holster. Some just wore the vest over the holster. In fact most vests didn't even have the holster on them. I never had a holster on any of mine through the years.
It was easier to just do it that way, rather than screw around with taking the shoulder holster off and on, and then having to keep track of the thing when it didn't have the pistol in it.
One thing about the shoulder holster I liked was it allowed you to keep the weapon on you without having to wear your LBE. It also made it easier to sit at a desk, etc.
I flew UH-1H's for six years in the 1st Cav at Hood, B-3/501st in ROK, and the 101st at Campbell. We turned in all the revolvers and were issued M9's at Campbell. It was the only NIB weapon I was ever issued.
