Validation of a flexfield on PO_LINES_ALL table

 I set up a flexfield on the PO_LINES_ALL table that I need to do validation on based on what the vendor is for the record that is pulled up The flexfield is visible in the Purchase order entry screen (POXPOEPO) and the Purchase Order Lines (POXPOVPO). When I set the validation up for one screen, using either the current form or current block I get an error on the other screen that it can not find the value. I have tried where clauses of

WHERE vendor_id = :find.vendor_id

WHERE vnedor_id = :Current_block.vendor_id

WHERE vendor_id = decode(:system.current_form, 'POXPOEPO', :lines_folder.vendor_id,