Uhm adding Indochina as as an "independent" country after Vichy falls, would open the theatre of Franco-Siamese war and could be puppeted by Japan later, even annexed when Germany falls. With the ai, there is no problem. Just set some locked japanese garrisons there and set Indochina expeditionary ratio to 0. But the player would use the French army vs allies.I think not.
The question is; would it solve AI issues.
We already had plans for a sort of fictional new japanese nation like KWA based in Vietnam to boost the AI for a war into india but it would add other problems (like troop transfer and possible exploits and Bugs with it).
Atm you have the issue of Nanking China being the one taking territory in Burma, receiving expeditionary troops from Japan an its minors, while Nanking China can hardly maintain that offensive with its limited ic. I have seen that when it's Japan who takes directly the land it works better.