I hope this is not late to submit this. I have one headcanon that they both have trouble understanding each others' culture. Which might lead to some arguments or misunderstanding. I thought it would be cute and funny that Vietnam would sleeps off her anger and act normal the next day while America just feels guilty the next day. It would also be adorable when they try really hard to understand the other person's culture.
Of course it’s not too late! That’s an adorable headcanon, anon!