Anyone who says the US "lost" the Vietnam War is either an edgy Yuropoor or a complete idiot. We did not lose, but we also didn't "win" either.
To understand what happened, you first need to realize that the point was never to unite the country. We had tried this in Korea and it turned out poorly for us, because it caused China to enter the war and almost led to WWIII.
This time around, when China and the USSR said that it would mean war if we directly invaded North Vietnam, we didn't feel like calling their bluff. This was the first recipe for disaster. It is impossible to cut off a communist insurgency in the South when the people supplying them and supporting them are untouchable. They hid in Cambodia, Laos, and North Vietnam because they knew we didn't want to "expand" the war.
Second, you must realize the "domino theory." At this point in history, communism was spreading fast throughout Asia. Alarmingly so. Many in the US believed that if we simply let Vietnam go down without intervention, next thing we knew, all of Southeast Asia and Oceania would be Red. I for one agree with that entirely.
So, we did the best we could. We wiped out thousands, maybe millions of the fuckers. We bombed the ever loving fuck out of their infrastructure. We even maid raids into Cambodia and Laos. It wasn't enough. So, we tried the only thing left. We trained the ARVN. And in many ways, they lived up to expectations. Many South Vietnamese didn't want to be commies. However, their leadership sucked ass, and that's what ultimately lost them the war after we left.
But did we accomplish our goals? Well, yes and no. On one hand, South Vietnam no longer exists and Saigon is called Ho Chi Minh City. On the other hand, after literally 30 years of war, other countries in the region saw what happens when half the country wants to be commies and half doesn't, and as such, communism pretty much didn't progress much past Indochina.
It was a tactical US failure, but a strategic victory.