a country can't fight another country's insurgencies for it.
^this^
The US simply couldn't win a full-scale war in Vietnam, because the will of the North Vietnamese would always be stronger than that of the American people. The North Vietnamese were fighting for the reunification of their nation, whereas the US was fighting to prop up an unstable regime an ocean away.
With that said, the US doesn't need to engage decisively. The US can deny the enemy victory by 'playing dirty'
•Dispense with large-scale search-and-destroy missions, free-fire zones, and the like. Instead, adopt the Combined Action Platoons across all of South Vietnam. IIRC, these units suffered 2x the average casualty rate while inflicting 8x the average number of casualties on the enemy, so they are 4x more effective. Plus, you can split the VC from their popular support where they'll whither on the vine
•Use Special Operations to cut off the Ho Chi Mhin Trail. Don't attack the personnel carrying the supplies, but rather call in airstrikes or artillery strikes to save putting American lives at risk. Don't try to completely destroy the trail (after all, it's just a trail through the jungle, not a rail or road network that can be bomber). Just keep the North from moving anything big down the trail. Keep it to a trickle that can be dealt with once it arrives in the RVN.
•In his book
The Mission, The Men, and Me, Pete Blaber argues the US should've made the ARVN a force similar to the VC: light, flexible, localized, rather than a conventional military that proved to be ill-suited for both counterinsurgency and conventional warfare. An ARVN designed to 'out-guerilla the guerillas' would enjoy just as much, if not more, local support than the VC, training and supplies from the bottomless wallet that is the US, and enough air and artillery support to deal with any tight spots
Not sure how viable a strategy based on out-guerilla-ing the guerillas would be in the US political and military order of the day, but it's interesting food for thought