You can just use po_vendors as there will be a synonym in apps anyhow.
select * from all_objects where object_name = 'PO_VENDORS'.
Check the Owner column to see which schema it belongs.
The "po_vendors" was a table in the PO schema in Oracle EBS 11i.
In R12, the suppliers are stored in "ap.ap_suppliers" instead.
The "po_vendors" is kept for backward compatibility as a view and it retrieves the data from ap_suppliers.
In R12.2.4 it is better to use apps.ap_supplies.
In 12.2.x, only objects owned by APPS can be accessed by custom code. because of object editioning, directly accessing any seeded objects in other seeded schemas is not supported and will lead to erroneous / unpredictable results.