I was wondering how many USB buses VIM/VIM2 have?

Specifically, is the two USB ports are sharing one bus (not talking about USB C/OTG)?

Yes, you are right.

CPU ---> USBHOST_A  ---> HUB IC  --------> USB Port1
                                 --------> USB Port2

This is explain the affect to the music quality when is usb music storage attached to one USB and DAC to another.