Actually, the REAL reason the South Vietnamese Catholics lost the war was because the guys in Congress decided to just cut any arms shipments to South Vietnam, many of whom KNEW they stood no chance. It had absolutely NOTHING to do with South Vietnam at all.
And for the record, Diem ATTEMPTED to placate the Buddhists, it was a radical sect of Buddhists who were causing all the trouble, many of whom had been Communist infiltrators. If anything, Diem was TOO capitulating for the Buddhists, too willing to back down (though to be fair to him, the foreign press composed of Americans certainly didn’t help in fanning the flames).
Sorry, it had NOTHING to do with Catholics ignoring Buddhists (and for the record, the Buddhists, while it’s true that they outnumbered Catholics, were not a majority, since they were tied with Confucians at most).