Answer:
The cause of colonialism was France desire to expand its territorial holdings in order to increase its level of wealth. Another cause was the fact that France had superior technology than Vietnam, which allowed it to conquer the country.
The benefit for France was rent extraction from Vietnamese land, in the form of natural resources. The benefit for Vientam was the build up of some infraestructure, and the establishment of a few political and social institutions. However, France benefited much more from the colonial relation than Vietnam did.