So ignoring the other faults of the Vietnam war : poor tactical and strategic planning, failure to deal with the enemy on a strategic level, the unfairness and cruelty of the draft : why fight over the territory at all?
As I understand it, Vietnam was mostly an agrarianism society of subsistence farmers, with some modest level of development in the major cities. It doesn’t have any strategic resources of note, nor did it have a notable GDP. As an ally to the United States in future conflicts, at the time it was worthless and it still is to this day.
So, why was Vietnam the “hill to die on” for the Pentagon? Fighting over Germany or Japan or even Taiwan or Israel : understandable. All are, both then and now, valuable advanced societies with the capability to be strong allies. In the case of Taiwan or Israel, they are substantially more powerful than their population or land mass dictates.
In fact, wouldn’t letting the Soviets have Vietnam have simply saddled their empire with another liability, similar to allowing them to occupy Afghanistan?