The Vendor field in the PO Purchase Order Entry form, header Info tab.

Enter the vendor (from the AP Vendors form) that is supplying the merchandise on this purchase order. If you do not know the vendor number, you can enter the vendor’s sort name to display the appropriate vendor number.

Note: If you enter a non-compliant vendor, the header displays a red warning in the header section of the form
You can change vendors on existing POs even after having applied invoices. This is to accommodate instances where your original supplier had to be replaced by another vendor.
Note: If you have received any items on the PO (via PO Receipts Entry), this field is disabled and cannot be changed.

PO Purchase Order Entry Form