Ok. Not going to question too much on that. I would easily select configuration over customization.
Now back to your question. I am not sure how much you know about Price Determination Enhancement. If no, you might want to refer to it.
IMG > Material Management > Purchasing > Conditions > Define Price Determination Process > Enhancement.
1. Assuming that condition type already configure to take consideration of the new Vendor (ZLIFNR).
2. You can extend the KOMP (PO Item) with Vendor Field (ZLIFNR). if it doesn't exist, update structure KOMG.
3. Go to EXIT_SAPLMEKO_002 to move the new field into E_KOMP.
That's all, the system will take care of the rest.
Now, your pricing routine (VOFM) will be able to access the field and you can do whatever calculation you need. Pricing Routine has to be assigned to the Calculation Schema to be effective.
Hope above helps. There is a lot of ground that needs to be covered here and I'm just giving some pointers.
Thanks
Leonard