Passport:
Passengers are required a passport for international travel valid during the duration of your trip, as you must present passport for check-in and immigration purposes.
Some countries require a certain length of your passport validation beyond your visit. So always check the valid date of your passport and the passport requirement of your visited countries.
When you depart from or arrive Vietnam you will need to present your passport, boarding pass and a completed Arrival - Departure Declaration at the passport control.

The identification form:
When traveling on domestic flights in Vietnam, the acceptable forms of photographic identification - all of which must be valid at the time of travel - are as below:
National Passport; or
Motor vehicle Driver's License issued under the law of the Vietnam; or
Aviation Security Identification Card issued by the operator of the aircraft or the operator of an airport in Vietnam; or
Vietnam Communist Party membership Card; or
Press Card; or
Parliament Card; or
Military Identification Card; or
Legal document from authorities to confirm that the passenger's Identification Card is lost or yet to be issued; or
Airlines’ Identification Card; or
Birth Certificate or equivalent.

Visa:
A visa is permission for a non-citizen to travel to a particular country. To find out if you are required a visa for travel to a particular country.

Arrival documents:
For international flights, when required, you will need to fill in and present an Arrival/Departure document at immigration counter.
For departure from Vietnam international airports, Arrival - Departure Declaration forms are available at the airport before proceeding to immigration counters. For arrival at Vietnam, you will be handed the form before your flight at check-in counter or during the flight from cabin crew.