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Unfortunately, I don't think there is enough demand in Vũng Tàu for it to be viable. All of the language schools in VT, and most of them nationwide, are focused on English classes for kids aged 3 to 10 and IELTS test prep for teens, and I think there's just one uni in VT,  Ba Ria Vung Tau University (BVU), which doesn't seem to offer French:

Moreover, the current regulations to work in VN are excessively onerous, and most likely, your age alone will disqualify from being able to work legally. In the not-so-distant past, you could probably have been fine discreetly offering private lessons (although with few clients), but these days you would run the risk of being fined and possibly "blacklisted".

There is Institut Francais in Việt Nam (HCMC, Ha Noi, Da Nang, and Hue):

They may be able to give you more information.

...Moreover, the current regulations to work in VN are excessively onerous,...

Agreed. See the post, "New Foreign English Teachers Requirements Decision 4159/QD-BGDDT"

...and most likely, your age alone will disqualify from being able to work legally...
    -@jayrozzetti23

Not so. There is no age limit imposed by VN immigration with respect to employment (in any field).

That said, there is certainly a bias shown by employers favouring the hiring of "younger" applicants.