Why no mixed people in Vietnam?

There are many Vietnamese-American or Vietnamese-French people in the world but very few reside in Vietnam. According to my tour guide, mixed Vietnamese-Americans were considered shameful back in the day due to the violent history of the US army in Vietnam. As a result, many of these kids were abandoned. But when Vietnamese citizens were unable to travel abroad due to government restrictions, they found use for the mixed kids. It was legal for anybody with mixed blood and their relatives to travel abroad (from my understanding it was established for these kids to be able to visit their birth parents). Because of this, many Vietnamese citizens would adopt these mixed kids as a ticket to go abroad. Now, most of the mixed population lives abroad.