Which Idea ,other than economic exploitation …

French colonization was not based only on economic exploitation but was also driven by the idea of a 'civilizing mission'.
Like the British in India, the French also claimed that they were bringing modern civilization to the Vietnamese. They took for granted that Europe had developed the most advanced civilization. So, they felt it was the duty of the Europeans to introduce these modern ideas to the colony even if this meant destroying local culture, religions, and tradition because these were seen as outdated and prevented modern development.
In order to consolidate their power over the Chinese who were powerfully influenced by their culture, the French dismantled the traditional educational system of the Chinese and established French schools for the Vietnamese. The French also needed an educated local labor force.