It was thousands of miles a way, heavily backed by the Soviets and Chinese and had overwhelmingly popular support as well as having a home guerrilla advantage in deep jungles

You cite these facts to explain the defeat, yet all of them were known or could reasonably be expected upon first entering the war. So according to you, the decision makers should have known that they would lose but they entered the war anyway and lost anyway. They miscalculated once, they may do it again.

Not to mention the evolution of weaponry, tech and tactics since the 1960s.

Be more specific.