In my opinion Marriage ceremony wouldn't count as a bona fida marriage. A actually legal Vietnamese marriage certification would count as bona fida. I had my marriage ceremony in Vietnam with my wife's family and friends. We didn't file any paperwork in Vietnam to register our marriage at the local government office. Once the wife arrived in US after 90 days we files for marriage license and officially got married in the US. Technically my wife isn't married in the eyes of the Vietnam government since paperwork wasn't filed there. She could marry another Vietnamese guy I suppose. Lol.
Don't overthink it. K-1 would be your option if not officially married in Vietnam.
At the k-1 visa interview they may ask you if you got engaged or had a ceremony. My wife answered that she was engaged and had a ceremony only. One requirement for k-1 visa is your Vietnamese wife submit a single status certificate saying she not married.
