So France obsesses harder and earlier about Alsace-Lorraine? (Actually, is the former even possible?) As it happens, I was thinking of this as being part of a larger TL where Germany works harder to keep friendlier relations with Britain, so that could actually work out for them longer term.
Think a lot more credits assigned there, a larger focus on the army compared to the navy, more fortifications...
Lots of credits for modernization of France's infrastructure
However also more private investments in Europe or South America
Now this is something I genuinely don't know anything about, and would be curious -- what cultural affect did their colonies in Indochina have on the French Third Republic? (I could probably give a decent answer on how Indonesia affected Dutch Culture, how Portuguese Africa molded Lisbon, and I expect everyone knows a thing or two about how Indian cuisine impacted Britain; but I'm drawing a blank on how what to look for in French Indochina's mark in Paris or the rest of France.)
Bit more long term, but there's definitely a large influence both ways, in cooking and vocabulary for Indochina. For Africa, it would be massive! Francafrican culture is heavily intertwined, just look at ethnicities in France today!
Since investments wouldn't be spread over the whole thing, you might get more money put in Cochinchina, ending up with a highly advanced economy with more settlers in if you have a proto industry. Since it's culturally distinct from the North (khmer vs viet), you might see the birth of a truly metis society, heavily frenchified
(Mind of, none of this means France completely loses Indochina in TTL's 1885 -- we've touched already on what the peace being more balanced, not a France-screw -- I'm just talking more generally about a related question.)
Another question is, if they lose then, what stops them trying again later? Avenge our fallen sons and what not. A properly prepared campaign against China would be devastating, and France would probably ask for more compensations. I doubt Vietnam could modernise much if France puts an embargo on it