In my experience, the east Asian diaspora populations in the US express a significant amount of solidarity with each other.

This is not remotely true in my experience. At best, it’s sorta true if you’re talking about 2nd+ generations. For the 1st generation, a lot of the country beefs like China vs Korea traveled with them. I’ve had Korean people say that parts of Manchuria should be part of Korea, Chinese people constantly shitting on Japan for what they did during WWII, and so on. A lot of the provincial beefs Chinese people have with each other also traveled with them, so northerners vs southerners, waishengren vs benshengren, the Mainland vs Taiwan vs Hong Kong. Pretty much every specific East Asian group has their own church or temple, so the South Korean Christians go to their own church, the Taiwanese benshengren go to their own church, the Hong Kongers go to their own temple. There isn’t a generic Asian church or temple that people of Asian descent go to.

Your experience is only Chinese and Vietnamese, and that honestly has more to do with the Chinese diaspora being traditionally from the South and part of the Vietnamese diaspora being Hoa, which is the ethnic Han diaspora in Vietnam that also came from the South. So, a Cantonese diaspora could get along with a Vietnamese diaspora with the Hoa/Cantonese Vietnamese acting as a bridge between the two communities. Mix in the more white-washed 2nd+ generation, and it’s not hard to see intermingling between the two communities with Canto-Viet families that are still pretty in touch with their Asian roots. But a bunch of Beijingers or Northeasterners isn’t going to mix well with the Vietnamese diaspora at all.