He'd be pretty old to still be on active duty. Vietnam ended in 1975, if he was 18 then, unlikely since by '75 only embassy guards and maybe a few advisors were still in Vietnam, he'd be 45, with 47 being a more reasonable minimum age for a Vietnam vet. 45 would be about the minimun age for a Vietnam era vet, if the guy was enlisted just soon enough to get the "alive in '75" ribbon/medal. (NDSM)
It wouldn't be all that old for a guardsman or reservist though.