Dual Nationality
Well, dual nationality can always happen; the true issue is recognition. If you hold citizenship in countries A and B, then A will only recognize your A citizenship and B, your B citizenship. What that means is that you cannot claim any privileges of B citizenship while in A and vice versa, e.g. if arrested in A, B's embassy will not lift much of a finger for you.
If you secure Vietnamese citizenship, that government has no power to revoke your French citizenship although it may require you to renounce it before naturalizing you. (I have no idea what Hanoi requires.)
French law, on the other hand, has provisions for you to recover French citizenship if you have already it in one way or another: in that case, you go for a "réintégration" and not a "naturalisation".
Good luck.