Actually no, we're a republic, not a democracy.Isn't America a democracy after all?
Well, yes and no. We achieved the Paris Accords, which were supposed to end the war, then took all our troops out and stopped defending the country. It would be more accurate to say we abandoned Vietnam, as we did Europe after WW I, with similar consequences. Note we are still in Europe, Japan, and S Korea today.You lost Vietnam, never mind how.
It's even harder to see how an unopposed Communist victory in SE Asia would have been better for America.It's very hard to deny that you'd be better off if you'd just told the French to F off back in 1950 when they came asking for help.
Had we not been fighting them in N Vietnam, those resources would likely have gone into another invasion somewhere else, perhaps of Korea again, and we might have lost Japan and Thailand as well -- assuming the same logic of "not worth fighting for" was applied in all those cases, it's practically inevitable. The Soviets were always expanding somewhere, virtually from inception. It never stopped until the 1980s, when our relatively cheap investment in Stinger shoulder-fired heatseeking missilies made Afghanistan very, very expensive for them, Central America went sour on Communism, and missile defense and the arms race was bankrupting the stagnant, ossifying evil empire even as America got a little richer every year.