Militarily the US were making the Vietcong and Vietminh their bitches, but as in support of my earlier point America lost a lot more by going into the war logistically and internationally just like Iraq and Afghanistan except this time theirs a nation waiting to'leapfrog' them.
We also sponsored dictators such as Diem to lead the south who were violent sociopaths that loved killing buddhists. In addition, our officers were inexperienced and the Geneva convention went out the window. So between always sporting Vietdong deuce-bags and committing atrocities we weren't going to win the hearts and minds of anyone.
The US brass made a shit load of bad decisions, from the minimalist attitude for taking hills and bunkers in the mountains to assuming that the Vietnamese wouldn't fight during Tet. Add to that an increasingly volatile movement of change back home and it wasn't much of a recipe for victory.
When you look at the total numbers the US military killed many more Vietnamese than they lost soldiers, destroyed the majority of the infrastructure in the North as well as in Cambodia and Laos, and manage to kill off hundreds of thousands of hectares of jungle and farmland. On paper it would look like a victory, except for one important thing. The Vietnamese never surrendered. So we just said "fuck it".
I respect the vietnamese. We totally destroyed their country, yet they seem to have bounced back just fine.