Because the US always needs something or wants to do something, And war atleast at the beginning always brings the nation together. So an unpopular prez ( Trump cough cough ) could unite the nation under war, Then since it would be a poorly thought out war we would get bogged down like Nam, And then we would have problem on the home front like political polzaration, Race, Or income inequlity and that would spilt the nation almost 50% / 50% and we would be almost in the same spot ( there are more reasons why like China propping up goverments in Africa but I don't want to talk about it. )
If there are any people in country's that are currently communist there are probably people who are whiling to rebel against the government "Viet cong" than we have a large civil war and this is bound to happen in many country's some of them with large allies "the US" that will fight with the non-rebellious side.
If we are talking about an unpopular war that no one is quite sure about why we are there, the we already have that in places such as Afghanistan and other spots in the middle east. And we are still doing the kind of boots on the ground fighting that has our young men in the trenches for no discernible reason.
Yes, there will be because history has a way of repeating its self. Look at what is going on with Russia and the Ukraine currently. I feel that this could be the next Vietnam war getting ready to happen again. Over what land and oil and gas. We need to learn to support each other and not attack each other.
Vietnam was a long and costly war where, quite frankly, the US was fighting shadows of a perceived enemy. Moreover, they were doing it inefficiently and at great cost to ourselves. Just from the strategic side of things, the US military was engaged in a doctrine called "Technowar" and relied upon methodologies that were simply incorrect for the enemy and the environment, physical and political, that it was being practiced in. With the increasing spread of information in the modern age, it's also my belief that people simply won't support a war conducted for such a spurious reason once they become aware of the situation.
I do not think that there will ever be a war like Vietnam again. I think that the United States of America learned a lot about that war. It wasn't a war about the strength of a country, it was a war about the politics of a country. I think that there will never be something like it again.