Enhance the process that creates the BP record to set either customer or vendor based on the context from which the BP was created. If it is created on the PO, Invoice(Vendor), etc windows, then it should be created as a Vendor and not a customer.
Run a complete cycle of all procurement processes from PO, to MR, to Vendor (Invoice) to Payment and make sure that if a BP is created it is set up as a vendor.
Run the same tests from the sales transactions to make sure these ones are still created as customers.
Attached you can find a patch for this feature.