We insist on you that if you have approval letter of immigration department, you are allowed to enter Vietnam.
Then the immigration department has save your information with a unique code on the system, because the code in approval letter is unique.
When you arrive Vietnam international airport, the immigration department officer will check the approval letter we sent you.
They will check on their system with uniqe code, and compare the information on system and your passport. It is is the same, you will get visa stamped and you are allowed to enter Vietnam.