You need to contact with the Department of Justice to have the registeration form. Furthermore, there are a lot of required documents when you decide to get married in Vietnam such as proofs of being single for both parties, health checked documents etc. so make sure that you know about these documents for best preparation.
Be aware of time if you are foreigners! As Vietnamese gorverment needs proofs of being single so maybe you must be back to your country and have your country gorverment confirmation. Moreover, the period of having documents checked at Department of Justice is around 25 days, waiting for the interview is 15 days and waiting for the final marriage registeration form is 5 days so couples should plan a firm schedule in advaced.
When your documents are confirmed and reviewed, the Department of Justice (DJ) will invite you for interview. In Vietnam, if the couples cannot answer clearly 7 questions the DJ staff give, they will be banned for marriage registeration for 6 months. Those questions are around the partner’s past, family, finance and characteristic.