No, you cannot. You are strongly advised to estimate the correct arrival date while applying because you are only permitted to enter Vietnam later than your entry visa date, not earlier than that. Otherwise, you will be denied for entry at Vietnam airport.

For example, if you apply for 1 month visa with the arrival date starting on 19^th March, you can enter and leave Vietnam any time you want during 19^th March – 19^th April. However, it is impossible to enter the country on 18^th March.