You need a Vietnamese Pilot's Licence in order to crew a Vietnamese aircraft.
When you arrive in Vietnam, you do a groundschool in HCMC, and a Vietnamese medical in Hanoi. They will then issue you a temporary Vietnamese pilot's licence based on your ICAO licence. It is valid for 6 months.
Before the 6 months are up, you must go back to Hanoi and write an "Air Law" exam. Once that is completed you will be issued a permanent Vietnamese Airline Transport Licence.
I am not sure if you need a valid medical when you go to Vietnam. You are supposed to have a current type rating, minimum 500 hours on type, and experience on type within the last 6 months.
I left Vietnam in 2006, so things may have changed, but I believe this is accurate.