There's a bit of a difference between winning a defensive war with guerilla tactics and winning a offensive war with guerilla tactics. And there's also the fact that Vietnam was supported by several communist regimes with resources and weapons.

The Vietcong were using guerilla tactics in a defensive manner and were supported by several larger countries. The war also took place in a dense unforgiving jungle. The US ultimately gave up when the US people didn't want to keep up a pointless war. Modern war has also changed.

Since then technology has vastly changed. Satellite imaging is vastly improved. Drones enable quick precise strikes. The US military also trains on US soil. You'd be hard pressed to find any location that gives you an advantage environmentally for your guerilla war like the Vietcong had. Step outside or move supplies and you risk being spotted by a satellite or drone. You could get hit by a missile through your window without any warning. And with how divided the US is now, chances are a good chunk of other US citizens would be helping the government.