Ingenious -- I wouldn't have thought of that. But it's good, in that it keeps the kids sharp in Viet while they're learning good English.
One of the mini-scandals of the NEA/Left's handling of immigration/nonassimilation is that Mexican kids living in the U.S. interior are speaking very bad Spanish, as well as bad/no English. These guys can't do anything right.
A Taiwanese guy I know who wants to retire in China (why, is his business, I guess) is a native Mandarin speaker. His folks weren't Taiwanese but mainlanders from northern China who retreated with the "Gearwheel Chinese" (the Kuomintang) in 1949. He told me ten years ago that he had found that his own command of the four-tone system of Mandarin had grown shaky during his previous 20 years in America (one of his reasons for wanting to reimmerse in Chinese), and that it was a source of considerable concern to him that his teenaged kids' command of Mandarin was pretty bad, especially the tonal system -- but then, Mandarin's a tough language, objectively considered. Nevertheless, he wanted them to be sharp in both American English and Mandarin.
Individuals benefit by being polyglot, but the society suffers when Barneyized Leftist morons try to make it polyglot.
That's the flip side. The English-speaking kids were benefiting by being immersed in a second language on the bus ride.