No, because the Catholics control the military, the few independent civil institutions, and have enough Buddhist allies to keep things under control. RVN often seemed like Pakistan IOTL- a military with a state. If you can get
Dr. Phan Huy Quat, a highly respected Buddhist constitutional expert, as Diem's PM, then a Buddhist will be in charge of the domestic scene. The US can "persuade" Diem to do that using aid money as leverage, worst case scenario. Without the malign influence of his brother and sister-in-law he won't be as paranoid as he was IOTL. Put
Cao Van Vien in charge as Army Chief of Staff or JCS chairman,
Ngo Quang Troung in charge of IV Corps in the Mekong, and
Le Minh Dao in reserve.
Do Cao Tri, though thoroughly corrupt, is an excellent armored commander- "Patton of the Parrot's Beak." There is plenty of good talent at the top of ARVN and among the NCOs. The problem is below the divisional level from O-1 to O-6, and since those are the officers who interact with villagers, it ruins COIN because they are essentially Mafia-style extortionists in uniform. Thus leaving plenty of sympathy for VC wherever they go. Thieu should also get a senior HQ posting- he's better as a staff officer than a field commander, though reasonably competent in the field as well. Get rid of Ky, the Vietnamese Chavez (albeit a rightist) because he's extremely dangerous. Popular with the peasants but a thoroughly corrupt, incompetent, drug-smuggling genereal.