The Vietnam war (also known as the Vietnam conflict) could not be won by the US alone without making china uncomfortable and starting world war three. I would like to clarify a win as the united states stopping the spread of communism from north vietnam to south vietnam. Now there are only three ways to accomplish that.
First possible way to win, south vietnam gets a president that could win an election against the north vietnamese leader. Keep in mind that the war between north and south vietnam started when the election to unite vietnam was cancelled by the president of south vietnam. Then the north declared war on the south in hope reuniting vietnam. So in theory if you had a really good president (very difficult task), then protected south vietnam long enough for the good president policies to work, and then got north vietnam to agree to a reelection, you could reunite vietnam under a capitalist presidents. Only problem is that there is a ton of things that can go wrong. We have a hard enough time getting a good president in the US let alone a country in civil war. Also you would need the policies to work, what if the policies don't work? Also it requires north vietnam to be willing to have another election in a middle of a civil war, after the first one was cancelled, and in a situation when they aren't going to win. So many things that could go wrong,.
Second possible way to win, getting north vietnam to accept the existence of south vietnam. During the vietnam war, north vietnam felt it was justified in fighting because south vietnam was corrupt. If you were to eliminate corruption in south vietnam and have it thrive, then it is possible that north vietnam would lose its willingness to fight south vietnam.
Third possible and most likely of the three to win the war, get south vietnam to a point that they can defend themselves with limited US support. This would require elimination of corruption in their army, as well as them having a wanting to stay capitalist.
I would just like to point out the mistakes the US made in the Vietnam war. One was the massive bombing campaign. Bombing the enemy's cities always increases the enemies moral, this was prevalent when germany bomb england, when the allies bombed germany, and when we bombed japan. Us getting japan to surrender by bombing them was a fluke, keep in mind that they were still willing to fight, the only reason they surrendered was because they feared we would do the equivalence of the firebombing campaign just with nukes. Another problem with the bombing campaign was that most of the time they were just wasting bombs on trees and dirt. While the US airpower was great when there was a standoff between the two armies, it did very little when used when we didn't know where the enemies were.