So the system of USB VID/PIDs as far as I understand it is to avoid conflicts between devices and the USB-IF is handling the distribution of these IDs and wants a fee if you need a VID and a corresponding PID block.
Now how is the situation looking if you have an enclosed device where systems within that device talk via USB but the device itself does not expose a USB port or advertise USB compliance or the use of USB in any way, it is just used for internal communication like you would use SPI or I2C.
Do I still need a VID/PID from the USB-IF? If yes is it allowed to then just use the prototype VID 6666?
The USB Host device in this system is running Linux. So a driver is in use, the end user just can't plug in any more USB devices to this host. I have the ability to chose the VID for every device connected to avoid any conflict.