RE: Vendor Bill Credits
If I understand correctly you are looking to write off some outstanding vendor bill credits that are no longer applicable. You have options:
(1) Generate dummy vendor bills that offset the bill credits, apply them via a $0 vendor payment. This will give you the credit to AP needed to settle the vendor credits. The debits would presumably align to the vendor credit for a net zero impact.
(2) Generate a journal entry to credit AP and debit whichever GL account you need to e.g. to reinstate the expense, post to a write off account, etc. then apply them via a $0 vendor payment.
There are alternatives, but I would recommend one of these options depending upon your internal processes.
I hope this helps!