Visa requirements

Hi. I am confused? My wife and I are flying to Australia via Vietnam , and returning via Vietnam . We arrive in Hanoi on 14th Feb, then leave for Sydney on the 17th Feb. We then return via Saigon on the 18th March, and leave for UK on the 23rd March. So we are only in Vietnam for 9 days total. But the gap between leaving and re entering is 30 days. It’s confusing as to whether 30 days is enough, or must it be more than 30 days. If you take the 17th February as our leaving day, but count that, its effectively 31 days before we re enter. Obviously if we can avoid the hefty fees visas it would be good, but we do not want to be held up at the borders for any infringements. Help!!