isn't this not just selecting the right conditions for a given variant at a certain vendor?
I don't think that those conditions actually control the source determination.
I did not yet come across such a requirement. We are ourselves a manufacturer of configurable materials, but rarely buy them, and if then usually from the same vendor. I guess this mainly a manual process using the desired vendor to tell purchasing from where it shall be procured.
Maybe you could include the vendor into the configuration. Still the buyer would need to look it up and SAP could not itself determine the vendor.