There are simply more Chinese so brain drain will be more pronounced. But I would also speculate that a larger percentage of Vietnamese are second generation or more. Very true in little Saigon. Poor Vietnamese who were born in the USA live in east San Jose, Chinese live in Cupertino. Very different levels of privilege.

That would only be true if the US allowed only the same number of immigrants from each country. I don't think that's been the policy historically. I'm not familiar with the current policy.

People immigrate for different reasons at different times. The causes combine with changing US immigration policies and the dynamics of preferential attachment to diaspora clusters to create distinct cohorts that have similar socioeconomic outcomes.

You see this in Orange county, too. Garden Grove and Westminster vs. Irvine.