If the US had the political spine to defend South Vietnam another 3 years, it would have seen a huge sea-change in the world situation.
Namely - in 1976 Mao tse dong died, along with Zhu De and Zhou Enlai. the Gang of Four was overthrown and that was the end of Mao’s politics and the start of practical rule in China. Who knows what accommodations could have been reached with Deng Xiaoping on Vietnam?
It was around that time that China was in a war with Vietnam. Traditionally, they hate each other.
I read an interesting article some time back that ending the bombing of the north saved their bacon.
General Giap (?) stated that the north would have totally collapsed if the bombing had continued another six months.
Not sure about the truth of the article but I definitely remember reading it.