Not my opinion, since I know nothing about the topic, but what I read on-line.

It only applies to Vietnamese ladies living in Vietnam. It's sort of like the phenomenon of the Russian bride we have in the US. Because the woman is from a country that is much worse off economically, a guy from the better-off country will be able to get a bride that would normally be way out of his class.

Didn't mean to sound insulting, sorry...