I'm not the first to weigh on this, nor will I be the last, but I really like this discussion and I just saw "Kong" so 'Nam is in my head right now. With a POD sometime between 1955-1975, win the Vietnam War for the USA by having a capitalist South Vietnamese state that lives on to this day.
Massive bonus points your scenario realistically achieves this after Diem's assassination.
So my two cents: IMHO, the biggest weakness of the US/SV were the extremely volatile internal politics of the SV government, especially following Diem's death. Coups, counter-coups, and constant civil unrest seemed to plague SV throughout the war (1963-1965 seems to have been particularly devastating). To me it seems the easiest path to victory address this problem far better than in OTL. How to achieve that? beats me.