Yes, Vietnam Visa On Arrival are always available at the airport 24/7. However, make sure you understand these below things:

1/ Vietnam Immigration Department is the Government Office, which issues Vietnam visa. This office is closed at Saturday, Sunday and Vietnam holidays.

2/ Although the Vietnam Immigration Department is off in specific days (mentioned above), the Visa On Arrival office at the airport always works 24/7. It means that you can get visa on arrival at any time you arrive when you had the visa approval letter already. The visa on arrival office are always opened (even in Saturday, Sunday and holidays).

3/ It is required to have a visa approval letter before flying to Vietnam. Without this letter, you cannot get visa on arrival. Please note!

4/ Visa on arrival is available at the Vietnamese international airports. You cannot get visa on arrival if you come through border or seaport.