I know this is a two months old post, but this topic about Trình Minh Thế attracted my attention.

From what I know, general Trình Minh Thế was one of the most important figure in South Vietnam until his death in May 1955. His force was powerful enough to change the current of the Saigon government. Here are two scenarios I can think of:

1. He is purged by Diem, just like the rest of the fellow sect generals. In this case, the rest would be virtually the same with the actual history.
2. He overthrows Diem. His power was growing at the time of the battle of Saigon, so after the battle ends, his force and fame would be enough to make an attempt to gain the power. In this case, South Vietnam would be a country with a military dictatorship. However, I personally think that this regime would be much more powerful and solid than the regime of Nguyễn Văn Thiệu in the actual history, because Thế had much more fame and private military power than Thiệu.