You do not need to provide the exact date of arrival in Vietnam, but the earliest date you expect to come to Vietnam. You can surely come to Vietnam after that date. However, please remember to leave Vietnam prior to the expiry date mentioned in the approval letter.
For instance, if you want to apply for a Vietnam visa for a period of 1 month from June 16, 2020, the visa will be valid from June 16, 2020 to July 16, 2020. You can come to Vietnam at any time between June 16 and July 16, and must leave Vietnam before July 16, 2020.
Note: The approval letter cannot be changed after being issued, so if you want to come to Vietnam earlier than the date mentioned on it, you need to request a new letter.