Asians I believe are also the youngest group in the country so that factors into the answer. There’s a notable history of racism against East Asians in the US going back to the 1800s, it’s possible that Vietnamese people are a bit inured to this and see themselves outside of those stereotypes.
Anecdotally, there’s one young Asian guy I canvassed once for the 2020 primary who came off very cynical and sort of teased me. It’s likely that those among them who are the type to cynically vote for Trump just don’t care in the first place. Like if you’re white you can be a groyper, he’s probably just disillusioned and a gamer.
Nah, us cynical Asians vote for Biden.