I don't understand why you don't take Vietnamese. Surely if it's your first language it'll be so easy for you! I know someone who was Hungarian and came 1st in the state in Hungarian Continuers I think. They did like no work in it too because they were fluent.
I say you take bio over any humanities, and I'd say business studies would be the easiest humanity for you (considering English is your second language). But really, all humanities typically involve lots of writing, so you'd need to improve your language skills which will most likely happen over the next two years...
Your English is extremely good for the amount of time you've been here! It's easy to understand what you are saying, but you have some grammar issues you'll have to improve on.