This one sure comes and goes. In the past we would have to delete the Primary Vendor entry in enter inventory and it would print the vendor part # on the order. I just tried another one and here is what I see.
 
Enter PO with a part that has a primary vendor. No Vendor part # prints. Edit the part and remove the primary vendor go back in the the po and enter another line item. Still no good. Delete the PO and make a new one, enter the same part # and Shazzaam prints the vendor part number.
 
Anyone else see this happen?
 
John