I did a lot of reading on this particular subject. An opinion which I think is the most correct is that the war was driven mostly by knee jerk reactions and whatever faulty data there was, was never considered.
LBJ didn't want to "lose" Vietnam. LBJ remembered Joseph McCarthy who accused the Truman administration of losing China to communism.
JFK and Ngo Dinh Diem were murdered in 63, Gulf of Tonkin incident in 64, and full US armed intervention by 65. This was a rollercoaster ride that the US military didn't plan for and didn't want.
McNamara picked Westmoreland to lead the military effort. It was a disaster. Their lack of a coherent strategy was doomed from the start. The body counts and #bullets per enemy killed was akin to rearranging the deck chairs on a sinking Titanic. On top of their minds was the Korean War, when the US military operated at China's border and China intervened militarily. Hence, US military land operations was limited to South Vietnam. There were no plans to invade North Vietnam.
Nixon and Kissinger decided the end game was to breakup the USSR-China alliance. With China on the US' side, the USA no longer had a national interest in what happens to Vietnam. The US left Vietnam in 73 and the country was overrun two years later.
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