Cochin China.
Massive natural resources, good money supply, lots of cultural mixing, liberal French administration, lots and lots of opportunities for some faction or group to start something weird but profitable.
That is a good point but not entirely accurate.
Saigon WAS a big town and fairly affluent in the XVIIIth century but by the French conquest, it doesn't seem to be that impressive anymore. I don't have exact numbers but it's not noted as a major population center by the French. I might be wrong.
Saigon was also quite separated from its traditional trade networks as it had been conquered from Cambodia. I doubt it was yet fully integrated with Vietnam's economy (which was oriented mostly toward the Hanoi delta) and Cambodia was really a dying beast.
The region was also fairly unstable by the time of the French conquest, due partly to resistance to the invader and anti-Christian persecutions.
However what I am telling you above might just be some reflection of Colonialist propaganda. It looks good if the Emperor before you is shown as very cruel and some kind of Asian Nero, hell-bent on killing Christians (Looking at you Tu Duc and Minh Mang).
Obviously that might be overplaying the Christian community and forgetting the actual disrupting role of Christians. It's a bit hard to blame the Vietnamese state for wanting to avoid having Christians when the reason they got invaded was for protection of the Christians...
Anyway, back to the topic.
With the Northern part of Vietnam, North of Hué, you have a very decent chance. By 1800-ish, the N'Guyen dynasty has re-affirmed control over the whole country and trade networks can start again. Hanoi is a stupidly rich and populated region. It was one of Asia's biggest city at the time, thanks to a very fertile delta.
You also had a clear bourgeoisie, a solid and educated administration, some artisans, high quality metalwork (and cannon foundries) AND decent trade integration through their ports.
On top of that, the founder of the dynasty, Gia Long, is perfectly aware of what a European army and a burgeoning industrial revolution can do, thanks to his association with the bishop Pigneau de Béhaine.
It also has a decent army and is not that easy to conquer. The French conquest wasn't exactly a walk in the park. The terrain is just awful for Europeans, and historically, the Viets use it a lot.
Now, the problem of Vietnam is clearly ideological. Vietnam historically is looking up to China senpai and won't do anything the Senpai doesn't do first. The N'Guyen dynasty is especially guilty of this.
So until they see some industrialisation in China (with the conflict against Confucean doctrine it includes) they most probably won't move.
Of course, there are ways to change that. Including a deeper conversion. Say if Gia Long is converted or allows wider proselytism in his kingdom. Or if Louis the XVI actually honours his promise of sending troups, you'd have an even further integrated Vietnam with great, relatively equal, ties with the West.
A retrained, re-equipped Vietnamese army is then used to pacify the country and, taking advantage of the massive troubles in China in that period, get some bits and pieces. Maybe even pushing to Canton? Who knows?