single entry or multiple entry visa

Dear Pierre

For French passport holders won't need a visa if staying for fewer than 15 days. However, the previous visit must be far from the next one at least 30 days. In addition, the passport must be valid at least 6 months. 

So in your case, you can apply for 1 month single entry visa for your first entry. And for the second entry, you can enter Vietnam under visa exemption that depends on your itinerary. 

Thanks

Anna Kim