Hi _Ferrari_
When USB as device, USB device gets the 5V from the host via VBUS. USB device judges whether VBUS has power to determine whether USB is plugged in. So we recomend user not disable VBUS to gurantee USB working normally.
If you don't care if there is a disconnect event or not, you can set FORCE_ VBUS bit to 1. It means VBUS is always connected. They don't need to connect VBUS pin from the connector to our USB_VBUS pin physically and the voltage level of VBUS won't matter. Thus I can't gurantee USB working stable. This is not our recommended design.
Have a nice day,
Jun Zhang