There should be a flag in the vendor file that allows a vendor to be marked as inactive so that new POs cannot be issued to the vendor. Currently the only way to prevent new POs is to delete the vendor, but then you loose part references for that vendor. Vendors might need to be disabled if a contract has expired or if an agreement directs purchasing to a different vendor.
I like this idea as well as for Customer File and Parts file. You could then use the flag to select to delete when appropriate. This would also allow the user to keep the active listings clean when selecting.