Well........The US did completely defeat and destroy the Vietcong. Then the NVA picked up the fighting. And the crap thing was for every advance/battle won, they gave it back a week later, then battled to take it back again a month later. A case of politicians dictating tactics and strategy to the people who know how to do it.
Though that war was 'lost' back home in the US. Seeing as it was a deeply unpopular war, we pulled out of Vietnam because of the promises politicians made forcing Ford to withdraw.
And the whole withdrawal was done badly as well. And as the support for the South was cut off entirely, and oil prices soaring meaning the South couldn't use their equipment, a whole lot of US hardware ended up in NVA hands.
We were certainly not a nice opponent to have during that time.