Need help for travel with 2 nationalities / 2 passports

Your child will need to EXIT and ENTER Vietnam using ONLY the Vietnamese passport. Similarly your child will need to EXIT and ENTER France using ONLY the EU passport. That's the way it works for all multiple/dual citizens anywhere.

Outside of Vietnam or any of the EU countries your child can enter or exit using whatever passport is most advantageous (or convenient).

Cheers,
James     Expat-blog Experts Team

Thank you so much James for your help !

Not sure if this still is the case nowadays, but I also hold two citizenship's. On one occasion I needed consular help but when I went to the consulate they said that in a foreign country I am considered citizen of the country under which nationality I entered the country and you can only use one passport to travel.

However, in your case I'd use the VN passport to enter VN but take the French one along as well because when exiting again, they may ask where your EU/Schengen visa to enter France is. Then you can show them the French passport. But keep in mind, you or your child will not be considered French citizen while in VN if traveling with the VN passport. Perhaps check that with the french Embassy or consulate.