I was just going to say no can do because there is no place in the data table to store a Last Cost by vendor but I just thought of a way to do it!
How about an SD-Q Default setting for "Auto-update vendor pricing with last cost?" If this is Y then the same data table used for PO-H would be updated when a PO is received (either create a new record or enter a new record if none exists for the item and vendor) UNLESS a record for the item and vendor already exists WITH quantity break pricing. Storing the info in that table means that nothing special has to be done in PO-A - the program already looks at that table first. And if items with quantity break pricing are skipped, you can still manually maintain the vendor pricing when they give quantity breaks but the others will be maintained automatically. How does that sound?