Vietnam aviation authorities have fined a passenger VND2 million (US$90) for using the life vest on a domestic flight without recommendations from the crew.
The man pulled the vest from under his seat on a Vietjet flight from Vinh in the central region to Ho Chi Minh City last week.
Life vests are equipped on aircraft for emergency situations. Using them otherwise is punished by a cash fine between VND1-3 million under Vietnamese laws.
Last month, a Vietnamese was also fined for removing a life vest on another Vietjet flight. Another one was fined for damaging a vest on another flight in May.
Critics say the penalties should be heavier because the passengers can put themselves in danger if they damage the vests and need them later.