In case you just wait at Tan Son Nhat airport (Ho Chi Minh City) in a short time, and you do not go out of the transit area, Vietnam visa application is not necessary.
However, if your waiting time at the airport takes for several hours and you want to go out of the transit area for some reason, like to visit somewhere or meet someone in Vietnam, you should apply for a Vietnam visa.
Apply for Vietnam visa online is a common method, after the processing time, you will get a Vietnam visa approval letter. Print this letter out and with your passport, you will have the Vietnam visa stamped on the passport when you arrival Ho Chi Minh.
Thus, you will be free to enjoy your short stay in Vietnam while awaiting for the next flight.