If you marry a Vietnamese national you are eligible for what is called a 5 Year Visa Waiver. It allows you to have an unlimited number of 90 day visits to Vietnam over the course of 5 years.
I was warned that some people had had trouble getting this while in Vietnam but I tried anyway. Big mistake. In HCM you need to go to the Immigration Office at 254 Nguyen Trai Street, District 1. It is by far the the most hostile Vietnamese government office I have encountered, ever. Minimal signage in English, few if any of the office personnel speak any foreign language, rather than help they prefer to complain to you at length and quite loudly. In the end I got some information but not the necessary forms. By then the one person in the office who hands out forms had left for the day (one hour earlier than everyone else). Lots more documents required in Vietnam than overseas and a trip back to your spouse's home province to get their family book.
I'll be applying overseas and recommend the same to anyone else in the same situation.