Yes, visa on arrival is quite legitimate. It is issued by Vietnam Immigration department and recognized as a legal document by the Vietnamese government. the IATA (International Air Traffic Association) has recognised visa on arrival. Check out the visa information sections on the following airline websites: Sky team, Oneworld and Star Alliance.

“Visa on arrival” is stated in the Immigration Laws of Vietnam Article 24, 1999, clause #6, Chapter II.

Other governments also recognize visa on arrival as a legitimate travel document, such as the U.S. Department of State & the British Embassy in Hanoi:

“In order to enter Vietnam, you will need a valid passport with at least six months validity remaining and a Vietnamese visa, a visa exemption document, or a written approval letter for a visa upon arrival. To obtain a written approval letter to enter, you must contact a travel agency prior to departure.”