Question

Why did French wanted to rule over Vietnam even it was ruled by the powerful empire of china?

Answer 1

The people of Vietnam came into conflict with the French in all areas of life. Though French control was most visible in military and economic spheres but the French tried to influence the culture of the Vietnamese.
Colonies were required to provide natural resources and other vital goods. Similar to other Western nations, France also thought it was supposed to bring the profits of civilisation to backward people