I wish I could give more detail, but this is about all I have... My father told us he served during Vietnam, but he never told us much, other than at one point, he served with the Green Berets (Emphasis on with, he was not a green beret, but he served with them....)
He is listed in the Veteran's Administration Death Index (BIRLS) with a service type of "other," but the US National Archives has no record of military service from my father.
Understanding there are numerous possibilities, including paper records being destroyed in a fire (I've seen that with military records in another part of my family tree...), I don't know who else to ask, or where else to look to find details on his service.
I would love to hear if anyone else has seen similar cases, and how you might have found answers.
UPDATE: I have submitted Form SF180 with the NPRC/National Archives, and was told they had no record of him. Yet, he is listed in the BIRLS, which, according to some of the recent comments, doesn't entirely make sense.