There is really no definitive database of Vietnam veterans - I think that the MilitaryUSA.com data base is one that was compiled by the VA for Agent Orange claims. It is comprized of names selected by the VA for a study and also from folks who filed with the VA for compensation. I'm a Vietnam Vet - but am not in the data base because I've never filed for compensation. The only reliable way to verify a Veterans status is to file an FOIA with NARA-St Louis and get a copy of the individuals record of assignments.
Thanks.