The Vietnamese were taken in as refugees; they didn’t have to follow the normal immigration procedures. This is much like how Ukranians are currently treated here - for now.
I was an 11 year old living in San Diego when Saigon fell. After the fall of Saigon, the USA immediately airlifted 130,000 South Vietnamese to the States, and they were followed by many, many more, the “boat people”, and later on Cambodians and Hmong. About 50,000 of the initial group were camped on the beach at Camp Pendelton north of San Diego. A lot of the initial group were high level military and political figures and their families who worked closely enough with the USA that the Viet Cong would have slaughtered them.
Shades of Afghanistan, but those people are getting shipped back, straight into the welcoming arms of the Taliban.
Many of the Vietnamese refugees ended up in Orange County, CA, where they were often treated terribly by the local whites, who saw them as intruders and were dumbass enough to call them “Japs.” But they persevered and developed several thriving cities in the area. And, the former South Vietnamese always were willing to help American Vietnam veterans who lived around there with quiet acts of charity, many of whom had basically been written off by the government. That continued at least through 2000, the last time I was in the area.
There were problems of course, as one would expect in any large group of traumatized people. Boys who didn’t adapt well ended up in gangs. The social practices of the Hmong also caused problems, especially marrying off the girls as very young teens and yanking them out of school.
But many thrived. I had the pleasure of teaching some of the sons and daughters of the refugees 30 years later at UC Irvine in the early 90s.
This is a completely different story than what’s going on now.