I spent many months in Vietnam and didn't see a single AR-15. I saw a lot of M-16’s but not a single AR. Are we talking about the same Vietnam war???
I was never there and I thank you for your service. But there is a lot of information available that says the AR15 was used, albeit not with much success.
In the early 1960's, U.S. Special Forces in Vietnam used the AR15 and it was given glowing reports. Although opposed by the Ordnance Corp, the Armalite AR-15 was adopted by the Secretary of Defense as the 5.56mm M16 rifle. In November 1965, the 1st Cavalry Division turned back North Vietnamese regulars in a savage battle in the Ia Drang Valley. LTC Harold G. Moore lauded the new M16 rifle his troops had used. "Brave soldiers and the M16 brought this victory," he declared.
The original name or designated by the military as M-16 were called by Eugene Stoner as select fire AR-15s.
http://duncanlong.com/science-fiction-fantasy-short-stories/ar15.htm
It the same as mil-spec M-9s sidearms which are really Bernetta 92fs.