How can I obtain a re-entry permit on a marriage visa before traveling to Vietnam?

You can get one at anytime at your local immigration office. Or you can get one before your flight at the airport - allow around 20 minutes to get one there.

If at an immigration office you’ll need to fill out a form called TM8. A passport photo, a copy of your passport, and a copy of your latest extension stamp. Cost is 1,000 baht for a single entry or 3,800 baht for a multiple entry.

At the airport you don’t need anything other than your passport but they usually charge an admin fee of 200 baht extra if you don’t have the form, photo and copies to hand.