While I'm aware of the widespread Buddhist opposition to the Catholic President Diem and his regime in South Vietnam, I feel like their role in history after 1963 isn't really mentioned much. I have a few questions
Any recommendations for books that go in-depth about South Vietnam's internal issues would also be much appreciated.
Take this with a grain of salt but, very broadly speaking, the subsequent leaders were mostly Catholics and Buddhists and none of them were nearly as ardently anti-Buddhist as Diễm (his policies were exacerbated by "Lady" Nhu, his sister-in-law, who was ardently Catholic). Officers were often coerced into "conversion" towards Catholicism in a bid to have a better shot at promotions and more privileges. No, the protesting did not cease immediately thereafter though one could say that the worst chapter in that saga was that of Diễm's. The infamous immolation of that one monk Quảng Đức took place in 1963, the very year Diễm and his brother Nhu were assassinated. The communists in Vietnam today are still suppressing religious activities, the assumption is that it's out of fear of uprisings. While the church enjoyed a privileged position during Diễm's reign, it's largely been stripped bare of that.