civil war
I know that you are defending Vietnam, but I'd still like to advise you not to use the word "civil war".
In the eyes of Vietnamese, this was an invasion from the US, they proped up a puppet regime against the will of the Vietnamese people, which includes the Southern Vietnamese people, who sarcrificed no less than the other regions in rebellion and revolution against the French colonizers. The US unlawfully denied the Southern Vietnamese people their freedom and independence.
What the English-speaking world call "the Vietnam War" is officially called ”the Resistant War against American imperialism to save the country" (kháng chiến chống đế quốc Mỹ cứu nước) or "the Second Indochina War" (Chiến tranh Đông Dương lần thứ 2) in Vietnam.
Edit: I forgot to add: The puppet regime was formed from a bunch of former French collaborators. The revolutionaries who fought against the French fought against them too. The puppet regime is illegitimate. And the war against them is, by definition, a war against foreign power, not a "civil war".