All non-scheduled flights operating to Vietnam require landing permits. Official permit lead time, per Civil Aviation Authority of Vietnam, is three working days. 
Validity of a landing permit is 24 hours for possible delay. Be aware that CAAV is closed on weekends.

Documentation requirements for permit

Official requirement by CAAV is: 

• Air operators certificate (AOC)
• Certificates of airworthiness and registration
• Worldwide insurance certificate

Other information required for permit application can be contact at your ground handling agent. Ground handling agent can submit the application and obtain permit on your behalf.

PPRs and Slot

Prior permissions required (PPRs) and Slots are not required for Vietnam. But for some airports that has few parking bays, CAAV may require operator to change the arrival/ departure time in order not to occupy the parking during local commercial aircrafts’ operations at that airport.

Ramp checks occur in Vietnam so be prepared

Airport authorities randomly check aircraft, to ensure that documents including landing permits are in order. Best practice is to have copies of all required documents ready and available. Also, your ground handler should have a copy of your landing permit, in case the crew does not have it.