Visa needed or not and if yes costs?

Hi, we are two couples arriving 7th of April 2016 at Hanoi airport, leaving via HCMC Airport at 00.15 the 22nd of April 2016. We stay all the period in Vietnam. As I red, for French people not exceeding 15 days stay, a visa is not required. The 22nd of April at 00.15, we will be surely for 1 or more hour in interational area at the airport, so we have left Vietnam, and haven't stayed not more than 15 days.
Is this reasoning correct then we do not need Visas or incorrect?
The second request is if visas are necessary, what are the costs in one package to get them and within witch delay?

Many thanks for your answers and advices.

Best regards

Jm Gross