PO-A allows entry of an item on a po to a vendor that is not valid for the item.
I do have the ISTECH option set to warn if an item is added to a PO that is not to a valid vendor.
 
We recently added a 'A' type item that did not list any vendors to a PO.
The PO was suppost to be a Service PO, but, was instead entered as a Purchase PO.
 
There was no warning that the item did not list this vendor.  The po got recieved,
which put the items instock, and then several days later, the work order
was received that put the items in stock again.
 
There should be a warning if a Purchase PO lists a Item Number that does not list the PO's vendor.
 
Additionally, if a comment is entered on a PO, it pops up a message that says that this item is not Approved for this vendor!!  Comments should always be accepted no matter which way the ISTECH option is set.