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.