i configure pci passthrough on my Proxmox and i saw in the GUI under the Advanced tab the Option to assign a Vendor / Device ID, but what is the usecase of this "feature" and in what format do i have to enter the Vendor / Device ID?
it's in the form of hexadecimal numbers, for example 0x1002 or 0x1022 in the beginning is for AMD devices (vendor ID). you can also look them up in /usr/share/misc/pci.ids. to see your devices ids' you can use lspci -nn. the last part that looks like [somenumber:somenumber] is the identifier, e.g. [10de:0040] would be NVIDIA GeForce 6800 Ultra, and would have the values 0x10de (vendor ID) and 0x0040 (device ID).