Only devices need a VID and PID not hosts.
The Vendor ID or VID is a 16-bit number which you have to buy from the USB Foundation. If you want to make USB device (and fully play by the rules) the VID identifies your organisation.
The Product ID or PID is also a 16-bit number but is under your control. When you purchase a VID you have the right to use that with every possible PID so this gives you 65536 possible VID:PID combinations. The intention is that a VID:PID combination should uniquely identify a particular poduct globally.
This guarantees that no PC can ever see two different devices with the same VID and PID as the PC uses these to identify the device otherwise a conflict could occur.
Note: Unlike a MAC-ID the purpose of a VID:PID combination is to uniquely identify a device type to the extent that the operating system knows what drivers to use it's not always necessary to be able to distinguish between two identical devices. For example two USB memory sticks has the computer not only knows what the device is it knows which port its plugged into.
When you plug a USB device in the OS asks the device for its VID and PID and gives the device an address which it will continue to use until the device is removed (or the computer switched off).
Once the PC knows the VID and PID it checks to see if it knows these already. If it does it loads the correct drivers for it; if not you are given the opportunity to install any relevant drivers. This process is called enumeration.
On the other hand a MAC-ID does not in its self provide any information about what a device is (computer, printer, router, etc.) but does uniquely identify a device: two identical PCs will have different MAC-IDs as if they both get plugged into the same Ethernet network it wont work correctly. Two identical USB memory sticks however will have identical VID and PID.
If I want to make and sell 1 million identical USB devices I only need to buy one VID. If I want to make and sell 1 million Ethernet devices I have to buy 1 million MAC-IDs