@Theo *********
for what purpose for the DTV visa will you be applying? Muay Thai training...
You hold a passport from France...
DO you have proof of having a bank account balance in your name only of 500,000 Thai baht or more for EACH of the 3 recent months ?
And how are you going to support yourself with the DTV visa?
Suggest you read the posts about getting the DTV visa at both Vietnam and Laos to see the ADDED documents requested by each to be submitted
WHY are you going to Vietnam?
I believe that Vientiane will be a BETTER choice.