Check for duplicate entries with number value and lookup value

I have a sharepoint list that requires a user to input a part number (text field) and a vendor (lookup field) along with some other fields. I want to allow the user to enter duplicates of either the part number or vendor (so enforce unique values will not work), but not allow duplicates of the combination of the part number and vendor. In other words, I want to allow only one part number entry per vendor and alert the user if the part number has already been entered for that vendor. Is there a way to accomplish this with a validation rule or basic functionality?