In order to get visa landing in Vietnam, you need to have the approval letter. This letter will show you the entry and exit date. You can just enter Vietnam from exactly or later than the entry date and have to exit some days before or exactly at the expired date.

Therefore, you can’t enter before the entry date because of its invalidity at this time.

In this case, if you need to enter Vietnam before the entry date, you need to do a new approval letter. No matter validity of the old approval letter, you still can re-apply a new one, and you just need to use the right one (which you need to enter at the date you want), and show it at the visa on arrival counter to get visa.