How did colonialism give rise to the Vietnam War?

Colonialism helped to give rise to the Vietnam War because
it made the Vietnamese hate the French and want to overthrow them.  The fact that the
main resistance to the French was done by the Vietminh gave that group (and its leader,
Ho Chi Minh) credibility among the Vietnamese people even after the French left and the
Americans came to support an independent South Vietnam.

In
the mid to late 1800s, France gradually came to own all of what was called French
Indochina (including Vietnam, Laos, and Cambodia).  The French colonial regime was
fairly oppressive and exploitative -- this was not a situation where the French were
trying to help modernize Vietnam so as to turn it over to its natives later on. 
Instead, the French were simply using Vietnam for their own
benefit.

Naturally, this made the Vietnamese very unhappy. 
They wanted to rule themselves, and anyone who stood up against the French looked good
to them.  The Vietminh's desire for freedom led to the conflict with the French after
WWII.  This conflict is what led to US involvement in Vietnam.