ROVN expected to be a clone of Republic of Korea; however, in ROK, there were a lot of nationalist leaders with a broad popular following, whereas in South Vietnam there weren't; instead, Vietnamese loyalties lay with Vietnamese nation created in May 1945 (by the Viet Minh). This had little to do with Communism, and more to do with the fact that there was nothing Vietnamese about the ROVN. It was not a republic, because there was no legitimate system of choosing the ruler, and no accountability to the people. And it wasn't Vietnamese.
Political opponents of the regime were routinely executed.
Eventually, US withdrew its support because there was an irrepressible conflict between the "realists" in the Pentagon, who understood that the approach to the war had to address widespread peonage, and the neoconservatives, who were ideological fanatics. The ROVN instantly collapsed, because it had no base of support left in the Vietnamese population.