There were indeed "pockets" of "free" areas within the northern parts of Vietnam. An interesting curiosity is that the co-regime of Vietnam lasted only until 1941 as Vichy essentially seceded all of their Vietnamese holdings to Japan by 1941. With regards to Vietnamese resistance, there was little if any effective movements until 1941 with the establishment of the Viet Minh by Ho Chi Minh on the northern Vietnamese-Chinese Border.
Though I would argue that Japan and Vichy control of Vietnam was not utterly overpowering, it's important to remember the revolutionary and nationalist movements by 1931 among them the Indochine Communist Party and the Republican Nationalists were heavily suppressed by the French government prior to the Japanese invasion. In fact outside of the ICP, the rest of the nationalist revolutionaries basically dissolved and even the ICP went dormant once the French control (prior to Vichy) increasingly suppressed any revolutionary movements.
Paradoxically by 1942 the Viet Minh gained the assistance of the OSS and the Free French. Though not entirely capable of fighting the Vichy-Japanese regime, the Viet Minh do create a front of resistance and slowly but surely create pockets of resistance in the North. Retrospectively I would say that the co-regime was strong enough to resist any revolutionary movements and control power until the fall of Imperial Japan. On the other hand, much of the work was already done by French colonial powers in the 1920s. Until Ho Chi Minh in 1941 formed the Viet Minh, much of the resistance within Vietnam already faced serious suppression.
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