Here in the State I live in, there are many mixed cultures and kids.
But it is the norm, so there is not as much conflict about it, as you are experiencing and Anthony.
Sure it can be a phase... but at the same time, it is about family cohesiveness... and being respectful among all those other things.
Anthony's biological Mom, is really HARMING his self-identity. It is racist. You did not say what Anthony's Uncle did.... was it a cultural misunderstanding? Or a plain outright wrongness? Keep in mind, that there are cultural differences, and if that family is traditional in that.
Next, your Husband is going to be hurt by Anthony's shunning of his heritage and culture. And, by his Ex-wife's racist view of it all. Her saying that the Vietnamese side is "weird" and "odd" is REALLY ALREADY harming Anthony's sense of identity. It really is sad.
Anthony's Dad, has to be an active part of correcting it, with his Ex, if possible. It really is teaching Anthony to hate himself... and he can't just change how he looks, nor who he is.
Yes, joining cultural groups can be very positive... because it will show Anthony and all of you (you/hubby), that there are many good people there and you could make some very good friends there, Anthony included. My family ourselves, joined a cultural group (my kids are mixed cultures) and we met MANY great positive fun people there who are now our friends. And, our kids go to learn about ALL cultures, and to feel happy about it.
Anthony, although young, is experiencing real harsh realities about racism and prejudice. Truly not good for his self-esteem. He should not be "taught" that his Vietnamese family is "weird" or not normal... but rather, he can be taught how loving they are and all about fun stuff about his culture. All this cultures.
And yes, his rejection of his Vietnamese side, is DIRECTLY a result of his Mom's bad-mouthing, and how things were handled per that situation with his Vietnamese Uncle. If it was a cultural difference, then Uncle as well has to be explained to, about what is allowed/not allowed, with Anthony. That is what any parent would do.
AND, your Husband is the Dad... HE has to be respected too, about his parenting and wishes for teaching his kids his language and culture. If not... NO ONE will respect "Dad" and this will create a bad "image" for that culture. Do you know what I mean?
All the best,
Susan