The approval letter is used for some requirements:

• You will be needing to present it for boarding the airplane.
• This is one of compulsory documents to get the visa stamp on arrival.

In this letter, you will see the entry date and the exit date. This is the period time you are allowed to enter and exit Vietnam. You can enter exactly at the entry date or later than the entry date showed on the approval letter, but surely you need to enter before the exit date.

You can also exit some days before its expiration, no matter how long you are going to stay or exit at exactly the exit date on the approval letter.