I know this might be a broad question and I will narrow it down if possible but i have a friend who is constantly arguing that America "won" the Vietnam War.
His reasoning is that America won due to the Peace Accord and the cease fire, even though the south later fell to the north in 1975. I would argue that in no way was it a "won" war because the goal was to prevent the spread of Communism and it didn't prevent that in the end.
It's a complicated answer and really depends more on the sort of question you're trying to answer.
The US didn't "lose" the war in the classic sense of the North Vietnamese were not able to force a conclusion favorable to their own war aims (unification of Vietnam), while the US war aims were broadly met (South Vietnam preserved) at the Paris Peace Accord.
Where it gets squirrelly is the US wanted out of the war, and was obviously looking to stay out of it as much as possible by that point.
So to this, some questions:
Let's just say after the Armistice in Korea that the UN pulled out leaving the South Koreans to stand on their own. The DPRK invades in 1960. Did the UN lose the war, did the DPRK defeat the UN, or is it something else?
South Vietnam sticks it out in 1975 narrowly, before ultimately falling in 1983. Did the US lose the war if the defeat is a decade after leaving?
Did the US expect North Vietnam to honor the Paris Peace Accords? If it did not, then it becomes fairly obvious the Paris Peace accords might have simply been a way to be defeated without being defeated on the battlefield.
Did the US government at the point of the Paris Peace Accords expect to have the ability to intervene at a future date to protect South Vietnam?
If North Vietnam did not come to the negotiating table and instead accepted being extensively bombed as the price of Vietnamese unification (ignoring the actual consequences of such a policy of course) what would have been the US response (withdrawal anyway, moving to an only air based campaign, invasion, whatever).
It's also important to ask why it matters if the US lost/won or what discussion is it being used within that requires more context. It's important for the Vietnamese national narrative to have defeated the US in the literal sense of having forced their will via national struggle. It's important for some segments of American society to have not "lost" the war and to shift blame elsewhere (while ignoring the reality that the whole point of the war was utterly dismantled within a very short window of the US departure).
This part is personal opinion but whatever, I contend that:
The US military campaign to prevent an invasion and occupation of South Vietnam ended "successfully" in that the US left of it's own accord, with a cease fire. The North Vietnamese militarily speaking were unable to force a conclusion favorable to their national aims with the US military in country (this distinction is important when dealing with people who try to portray the US as having been defeated by tunnel networks and jungle snipers or something).
The US as a matter of national policy, nation building, foreign internal defense programs and goals lost the Vietnam war in as far as it was unable to make South Vietnam "work" well enough to build a functioning government that served it's people, was able to defend itself, and stand on it's own.
Ultimately it's the second point that has the most relevance.
With that said, as simply an interesting counter-factual though, imagine a history in which the US did not go through major internal strife during the later 60's and 70's. Then contrast this with the US support of the Republic of Korea post war, and the struggles of the ROK to become a functional state as actually happened.
Basically my contention is that the lack of 25th ID on the DMZ between the Vietnams in 1976 has less to do with Vietnam as a special military problem and more to do with what was wrong with America at the time.