You do not have to provide us with your exact arrival date, but the earliest date that you plan to arrive in Vietnam. You can totally arrive after that date. However, please note that you need to leave Vietnam prior to the expiry date mentioned on the approval letter.
For instance: If you wish to get a 1-month Vietnam visa and submit the arrival date as Feb 1, 2019, the visa approval letter will be valid from Feb 1, 2019 – Mar 1, 2019. You could arrive in Vietnam at any time between Feb 1 and Mar 1, yet must leave the country by Mar 1, 2019.
Note: The approval letter could not be changed after being issued; hence, if you wish to arrive earlier than the date mentioned on the letter, you have to apply for a new approval letter in order to update the arrival date.