Go Tell the Spartans with Burt Lancaster is a fave of mine. It’s low budget, and very cynical (more akin to films like The Deer Hunter than The Green Berets). Still, Lancaster is excellent, and it’s the only film I know of that deals with the early years of Vietnam.
Here’s a clip of Lancaster talking with a psych warfare type.
http://www.youtube.com/watch?v=UF4-gqOW1_E
Er, profanity at the link, btw.
Check out Marshall Thompson’s A Yank in Vietnam