The historical problem with tracing the war back to this first moment of Franco-Vietnamese contact is that it would be teleological. A teleological history is pre-occupied with the end, in this case the Vietnam War, and views all events and evidence as leading to that outcome. The problem is that it erases all historical contingency, the many multiple ways events could have progressed. In this case, the colonization of Cochinchina only took place five decades later, and it was largely the result of freewheeling French admirals going beyond their orders.

I could not follow the idea in your second to last sentence. South Vietnam was not a colonial territory. It only came into existence in 1955, after France had agreed to transfer all sovereignty back to the Vietnamese. Prior to that, Vietnam was divided between two Vietnamese governments, the non-communist French sponsored State of Vietnam which controlled the urban centers and fought with France to defeat the communist Democratic Republic of Vietnam in the countryside. This division was in both the north and south of Vietnam, for example the State of Vietnam controlled Hanoi, Hai Phong, etc.

The causes of the Vietnam War as you mean here are connected to the competition between Vietnamese over what type post-colonial state they would have, and who would control it. And of course, in the Cold War environment, powerful international sponsors had their own agendas in supporting the opposing factions.