When immigrants were coming to this country from Viet Nam, the very next generation were winning the spelling bees. Both of my parents had a background of people who came to this country from non-English speaking countries. When I asked my father why we didn't speak any German, he explained that he once asked his father that and was told "We live in America, you speak English".
Two lessons: the first, that the Vietnamese (and most Asians) understand that education and being able to speak the language well, are steps up the ladder of success. Two: When you come to the United States, you should learn the language in order to fit in and then success follows. Simple lessons that we now actively work against in this society because we allowed the lefties to be in charge of education and the media.
I agree that Vietnamese have done an excellent job in assimilating. Much of it has to do with their culture, family values, and respect for an education as a means to achieve success.
The Vietnamese also did not have the same kind of support structure that Hispanics have in terms of being able to function without mastering English. Spanish has become the de facto second language of this country. And I would not discount the impact of distance, i.e., the Pacific Ocean, that makes it more of a distinct break from the their old country and becoming an American. And Vietnam is still under Communist rule. This is not the case for Mexicans and Latin Americans who can more easily maintain family and other ties.
And the wave of Vietnamese immigration is essentially over while Hispanic immigration is continuous, persistent, and massive. Two-thirds of the 1.2 million legal immigrants into the US come from Latin America and the overwhelming majority of the 500,000 illegal aliens who enter each year are from Latin America. By 2050 one in three residents of this country will be Hispanic.