No, we do not. But it would be better if you could provide as accurate as possible for your sake, because your duration of stay in Vietnam (noted in your visa) will be counted from the arrival date you provide. You can arrive and exit on any date within this duration. So, if you are not quite sure about your date of arrival, just choose some day earlier than your planned arrival date.
For example, if your arrival date is 1st January, 2012 for 1 month single entry, your duration of stay will be from 1stJanuary, 2012 till 1st February, 2012 (i.e, entry date is 1 January and exit date is 1 February). Then, you could enter Vietnam on any date after 1st January, and leave Vietnam on any date before 1st February.