To marry someone in Vietnam, you have to prove that you are eligible to marry with the Embassy of Vietnam. Documents needed include:
1) an application for marriage registration
2) affidavit of single status
3) certificate of no-marriage records issued by the state or county Vital Statistics Office
4) certificate of good mental health
5) biographical information sheet.
Please note that going to the doctor for a general check-up will not suffice for the certificate of good mental health, the information must be presented on a specific form.
If you have been in a previous marriage you must submit a copy of the divorce decree.