Is there Vietnamese in the SFV?
Yes. Mostly around Reseda and Sherman Way, though there are two places at Victory and Sepulveda (Pho So 1, next to 99 Ranch, is better than Pho 999), and one place (Pho 999) at Coldwater Canyon and Sherman Way.
The only banh mi shops in the Valley are at Reseda and Sherman Way. One's called "Saigon Sandwich", I think.
One of the places that has been mentioned favorably on the board before is Pho So 1 (I haven't been).
6450 Sepulveda Blvd
Van Nuys, CA 91411
I also have seen more than a few mentions of banh mi places on Sherman Way in Reseda but I am not recalling any names. Hopefully someone else will chime in here.