Honestly, I see a surviving South Vietnam (or unified Republic of Vietnam) as more of a Thailand-like state, rather than as a South Korea/Taiwan analogy, due to the fact that the Republic, after Diem's death, became divided by various generals who wished to rule Vietnam. And besides, Thailand today is capitalist. What say that the Cong Hoa (Vietnamese for Republic) wouldn't be the same?
Another PoD would be to prevent Duy Tan from dying. (
https://en.wikipedia.org/wiki/Duy_Tân) Duy Tan was an emperor of the Nguyen dynasty, but was renowned in Vietnam for, along with his father for resisting the French, as opposed to other Nguyen emperors like Khai Dinh who were effectively French puppets. Duy Tan was educated in the West (noticeably after being exiled to Reunion) and, after World War 2, was on his way to Vietnam to talk some sense into the chaotic situation there, since many Vietnamese respected him. His death in a plane crash in Central Africa marked the beginning of the end for Vietnam, as the only other figure who had that much popularity amongst the people was Ho Chi Minh.