From Wikipedia....
On September 1940, an accord was signed by Japan and the Vichy French administrators of French Indochina.[1] This accord granted various basing and transit rights also limited to 6000[2] the number of Axis troops which could be stationed in Indochina, and set an overall cap of 25,000[3] on the total number of troops that, including basing and transit, could be in the colony at any given time. In addition, the final article of the agreement barred all Axis land, air, and naval forces from Indochinese territory except as explicitly authorized in the preceding articles.[4] Exceptions to these agreements would require Vichy approval
As the French retreated southward, the Japanese encouraged Vietnamese troops to support the invasion. The communists in the Bac Son district border area moved to take advantage of the situation, organizing self-defense units and establishing a revolutionary administration on September 27, 1940. The French protested to the Japanese, however, and a cease-fire was arranged whereby the French forces returned to their posts and promptly put down all insurrection. Most of the communist forces in Tonkin were able to retreat to the mountains. In similar short-lived uprisings that took place in the Plain of Reeds area of Cochinchina, however, the communist rebel forces had nowhere to retreat and most were destroyed by the French
But WI the Free French encouraged the Military Units there to resist the Japanese Invasion ?
Or The French decided to set aside their differences with the Viet Minh and utilize them to resist the Japanese Invasion ? Or What if Roosevelt and Churchill had pressured the Free French to go easy on the Vietnamese Populace and make moves to grant them Autonomy to win over their support ?
Or the Japanese use heavy handed tactics against both the French and Viet Minh forces causing them to ally against the Japs ?
And would a Japanese Invasion of Indo-China make the Brits, Dutch, and US sit up and strengthen their defences especially in seeing how the Japanese would inflict atrocities against French, Asian, Civilian, and POW's ?
Also would a prolonged subjacation of French Indo-China severly affect the Japanese timetable for conquest especially if British & American Military "Observers & Volunteers" assisted the French & Viet Minh ?