In addition to the other advice:
Every table should have a single primary key and that should probably be an autonumber. If you need to enforce business rules such as making the combination of VendorID + PartID unique, do that by using compound unique indexes. You will need to make multi-column indexes using the indexes dialog. You CANNOT make them by using the indexed property on a field.
Once you have a single unique iD for the combination of PartID + VendorID, you will be able to use a combo box to select an item. Comboboxes do not work when the table they are based on has a multi-column primary key. I would sort the combo by PartNumber and then VendorName since the user will know the part number but might not know the vendor. That will filter the list so you can pick what you want.