The Vietnam application form asks for you to enter your 'Proposed date of entry' The visa will be valid from this date. You can make your initial entry into Vietnam on or after that date. The Date of Exit is not as critical to be exact as the visa is not date specific. The visa will be valid for one month, three months, or one year, regardless of your date of departure from Vietnam.
It is absolutely critical is to make sure to request a multiple entry visa to allow you to re enter. Please make sure to request a duration of visa that is valid beyond all your entries into Vietnam. You do not have to enter your date of re entries into Vietnam on your application form or the date of subsequent departures.