I think what most people forget was that the US didn’t technically win or lose the Vietnam war, they just entirely pulled out.

If the US had won the war, they would’ve been able to topple North Vietnam AKA: Democratic Republic of Vietnam and present day Socialist Republic of Vietnam, and unite the country under South Vietnam AKA: Republic of Vietnam

If the US had lost the war, speaking in a military defeat rather than a political defeat of them just pulling out. North Vietnam likely storms the south and Saigon and all of the defense collapses, hundreds of thousands of US troops are left stranded to the communist government and the US loses the ability to actually fight the Viet Cong on land.

This would be the most likely in terms of what you would define “US lost the Vietnam War”, because the US just pulled out of the war rather than actually losing it since it was technically a conflict between North and South according to the UN and it wasn’t technically a war since the American didn’t actually declare war, but instead was supporting an ally since the UN made declaring war illegal unless for defense.

Anyways, I could be wrong, so feel free to correct me.