The Vendor field on the AP Vendors form, Duplicate Vendors tab.

Enter the vendor to designate as a duplicate of the current vendor. Press F4 for a list of valid vendors.
Note: If open invoices exist for the duplicate vendor, the system displays a message that you cannot merge a vendor with open invoices, and will not save the record. You must close the invoices before you can add the vendor as a duplicate here.

Once you save the record, the system marks this vendor as inactive and displays a message on the vendor's record indicating it is inactive and a duplicate of the specified parent vendor.