So, sources I've found on the Indochina conflicts have said both 'occupying Japanese from WWII taught natives methods to resist occupation and guerilla tactics as a final F-you to the French/Allies' and 'Japanese remnant forces fought with the French during the conflicts', and I'm not sure if one was wrong or forces did both or what. What role did Japanese forces still in Indochina from the occupation play in the later Indochina war(s)?
The answer is both statements are correct, remnant Japanese forces within Indochina had played roles on both sides of the conflict.
Japanese with the French: Most Japanese remnants had left back to japan after the end of WWII however like some Japanese who refused to surrender some Japanese had stayed to either continue their lives in the area they had been in or like in Indochina some Japanese forces had fought alongside and even trained members of the Viet Minh to fight the French forces in Indochina.
Japanese with the Viet Minh: The Japanese remnants at the end of WWII who had not repatriated back to Japan after the surrender in 1945 had some roles in helping the French during the conflict, the role that comes to mind to for me is the one of guard, i know that some former Japanese soldiers had been chosen to be used as guards at some points within Indochina for the french.
if i can find the pictures of the roles of which i know a few of the guards exist then i will post them and when i can source some accurate numbers of former Japanese soldiers in Indochina post 1945 then i will update this post.