Here's a factoid I recently learned: something like 80+% of Vietnamese refugees were ethnic Chinese (Hoa people), despite constituting a minority of the population from which they fled. Rarely discussed in American history, except perhaps in in-depth treatises, is that ethnic Chinese were the merchant and ruling class in Vietnam for generations, including during colonial periods. That's why the French favored them. It's a classic colonial/imperial techniques to favor a minority group, though AFAIU France basically kept class divisions as they found them. And such minority groups tend to disfavor complete independence as it leaves them more vulnerable. Indeed, continued ethnic conflict after the U.S. departed instigated China to invade Vietnam a few years later.
If the U.S. had fully appreciated the ethnic divisions, it may have made different decisions. State-side the choice to prop up the wealthy, Catholic Diem is well known, but the usual characterizations of that choice completely gloss over the deep and substantial ethnic divisions. (Almost all Vietnamese Catholics were ethnic Chinese, though only are minority of ethnic Chinese were Catholic.) They make the U.S. seem merely tone deaf as opposed to tragically mistaken about the underlying social dynamics. The push for socialism in Vietnam wasn't about political ideology; it was about wrestling control of capital away from a discrete minority ethnic group. Those ethnic enmities ran deep. Without pursuing some of the social reforms sought by Ho Chi Minh a U.S.-backed South Vietnam was always destined to fail.
The irony is that as Vietnam has modernized the same social dynamics have crept back. Ethnic Chinese once again control much (or most?) of the private assets in Vietnam. I have no idea if the ethnic animus has returned to its prior levels, though. Maybe it doesn't matter as much anymore.