The OP asked how America could win. I took that to mean how the war could be won militarily, not whether it was genocide or not.
Unlike many here, who were not even born during the Vietnam War, I was almost old enough to be drafted for it. I can remember hearing the count of American dead every week on the news, and in 1968 and 1969, it upset me greatly. I liked to hear about the high numbers of dead VC and NVA, [I know now about how inflated the body counts were] they were the enemy, the bad guys.
I am also aware about how corrupt the South Vietnamese govt. was, and that ARVN was mostly useless, with poorly led, unmotivated, and often cowardly soldiers.
If one goes on the assumption that communist North Vietnam was 'Evil', in the way that Nazi Germany and Imperial Japan were 'evil', why was it immoral to bomb North Vietnam back to the stone age, when it was apparently OK to do it to Germany and Japan? Or were they due some special consideration that the Germans and Japanese were not?
If they were going to go to war at all, they needed to go in to win outright, which meant treating North Vietnam's cities no different to Dresden, Hamburg, Tokyo, etc. Or they should have stayed out, and just let South Vietnam go. And it is very easy to say all that now, decades later. I'm aware of the domino theory, and the fear of China getting involved, and all that.