At the leaving counter desk, show your passport, Vietnam visa, and boarding card to the check-in clerk in order for controlling and stamping confirmation.

Show the Vietnam visa, passport with the stamping mark, boarding card, and luggage at the security control to gain permission through the airport departure lounge.

Carefully follow the instructions of the airport staff, then queue at the regulative ports (which are accurately informed in the boarding card). After remaining seated, please strictly turn off all automatic devices and get ready for the flight.
Source: https://entryvietnamvisa.com