Well, if he enlisted in the Marine Corps reserves, it probably is fair game to say he served during the Vietname era. It might even be true that he was subject to taunts and abuse, since just being in uniform in public during that period could result in that. [Actually, I experienced the same thing when I was in ROTC] But saying he served in Vietnam is just wrong and an AG (of all people) should know that.
Serving during the Vietnam era is a huge difference then actually serving in Vietnam. My husband did two tours in Vietnam and he volunteered for both