The US would have "won" (I put it in quotation marks because it was never achieving much but human suffering) if it had stayed in just a bit longer. The last major NVA offensive basically used up their remaining reserves. Mistakes were also made far earlier that if avoided would have let the US win in the early years.
Sorry for OT, but a fairly good objective British documentary on the matter is Battlefield Vietnam:
Every US president was privately told by the CIA the US could not be win. Declassified documents show this.
"Strange game. The only winning move is not to play"
--Joshua the computer in the movie, War Games
