While there are some debates as to the amount of former Japanese soldiers who remained in Indochina after WWII, there is no doubt about the capacity in which they played in the still young Viet Minh at the start of the French Indochina War (1946). As the extract above tells us, they played the parts of instructors, NCO's and other, smaller but needed roles in an army that had never properly been battle tested before on a larger scale. While the Japanese involvement was important in getting things started (any experienced military instructor can do wonders with green recruits, of which there were plenty in this case), they should not be credited with the tactics that would make the Viet Minh infamous throughout this particular war.

A far larger and more important role was played by the Chinese, starting in 1950 and forward which would essentially push the Viet Minh into a much more capable fighting force than it had been before. The older cadre's (including General Vo Nguyen Giap) experience with the communist Chinese war against the Japanese during WWII laid the groundwork of the tactics that would be used in the later war and Giap's overall strategy was based on Mao Zedong's own principles for guerrilla warfare - albeit very much adapted and modified to the war in Indochina. China also sent their own instructors and experienced commanders from the war against the Japanese and the Chinese Civil War to serve in Indochina alongside Vietnamese troops. Men such as General Chen Geng who served in the capacity as advisor in 1950 was crucial in formulating the early important strategy in breaking out from the Viet Bac, the Viet Minh stronghold close to the Chinese border and to get out and actively fight the French. The CMAG (Chinese Military Advisory Group) was also formed with this in mind and would provide advisors down to battalion level.

To sum it up, the Japanese soldiers who served with the Viet Minh in the French Indochina War gave some valuable knowledge to the troops they educated and to the units they served in, but did not shape nor contribute to the understanding and expertise that the Viet Minh would one day have in guerrilla warfare. While these soldiers would have been experienced in jungle warfare, they were crucial in giving these young and inexperienced soldiers their first taste of something resembling military training. However, the "personality" and force of the Viet Minh would come over years of experience and the Chinese were very important in creating this. The Chinese had not only more men involved than the far lesser amount of Japanese ex-soldiers but did also extend their expertise up to General Giap himself as well as to provide the guerrilla based Viet Minh with weapons and supplies which would eventually turn it into a conventional army that would one day beat the French Army at its own game.

Sources:

China and the First Indo-China War, 1950-54 by Chen Jian in The China Quarterly, No. 133 (Mar., 1993), pp. 85-110

China and the Vietnam Wars, 1950-1975 by Qiang Zhai.