At 7 p.m. on Tuesday a flight attendant had found the 40-year-old man smoking inside a restroom, setting off the smoke alarm, and reported to the captain, the Vietnam News Agency reported.
Smoking, including electronic cigarettes, is banned on all domestic flights in Vietnam.
Passengers are warned not to smoke, but some still try to sneak a puff in a toilet, ignoring the ubiquitous no-smoking signs.
Smoking is one of many flight safety violations reported in Vietnam in recent years.