Even after the Japanese occupied French Indochina, it still remained nominally under Vichy French rule, it wasn't until near the end of the war in 1945 that Japan dissolved it into various separate and "independent" puppet states when the Axis in Europe collapsed, like Vietnam, Cambodia, and Laos (I will make a separate post only to ask about Laos soon).
Japan made a huge deal with propaganda efforts about their Greater East Asia sphere thing, an imperialist project disguised under the promise of getting Asia rid of Western colonialism, but Vichy-ruled French Indochina itself was not interestingly not included on it.
Since the Japanese (and the rest of the Axis) were so faithful that victory over the allies was inevitable and Asia would be theirs sooner or later, what did they plan to do with Indochina after their planned victory? would they force the French to give up Indochina anyways, or would they allow the French to control it?
I'm getting some of my info from this not-so-well sourced article on Wikipedia about the GEACPS, where it mentions that Japan would divide French Indochina under the "Kingdom of Annam" compromised of Vietnam and Laos, and Cambodia.
They would do what they did to other states. Japanisation of everything. When they conquered Singapore and Malaya, the Japanese immediately rounded up the Chinese (that supported the mainland) and start their mass execution.
If they’re not doing that, they would change everything to Japanese. Japanese signs, banning local language and force Japanese language schools, forced introduction of Japanese culture, worship the emperor. My grandma knew how to speak Japanese when we visited Japan.