No, it is not necessary to provide us the exact entry date to Vietnam.
The date of arrival you provide should be the earliest date you plan to arrive in Vietnam because you cannot arrive in Vietnam before that date but you can arrive later than that date once it is granted in your visa approval letter.
The exit date does not need to be exactly also. We just pay attention on your entry date Vietnam and the type of Visa to process your visa application.
It means, your Visa is 1 month, no matter the exit date you choose, your Visa will be valid for 1 month from the entry date you applied.
Note: The visa approval letter cannot be amended once it is issued, so in case you wish to arrive earlier than the date stated on the approval letter, you are required to apply for a new letter to update the date of arrival.