The U.S. army, while preventing the North from encroaching, were effectively sitting ducks and as the casualties grew so did the anti-war tide at home, until they pulled out and the south was left to their demise. The North, then separated when the U.S. arrived, bordered China, their arms supplier, and had thick jungle cover from the Ho Chi Min trail all the way to the south. While the U.S. took some pot shots at the North through bombing missions they "officially" were never invading.
This was an impossible situation from the beginning.
> South America guerrillas are unbeatable.
Bolivian insurgency failed, Guevara died. They don't always win.
The thing of it is the arms involved at the outset are irrelevant. You can't coerce a population largely unwilling to be controlled. Arms (including bombing devices) inevitably make their way into peoples hands. In the event of martial law things would fall into such disarray it's unsustainable.
I take issue with the idea that a population needs to be armed a priori with a specific set of weapons in order to repel a "tyranical government", which is a kind of boogeyman idea if I ever heard one. A tyrannical govt, comprised of a minority of people, versus the U.S. scenario is not a likely one. You'd sooner see a civil war, and even that's incredibly unlikely. But then I suppose you'd see gun regulation as evidence of tyranny.
The most tyrannical govt I can think of is the Chinese one and they have popular favor with their people. The HK situation shows that popular dissent is powerful with or without weapons.