An A/P aging shows amounts still owed to each vendor by days past due. Use the report's as-of date and each bill's due date, not the date the bill was entered. A bill due today belongs in Current. Partial payments reduce the open amount before it is assigned to a bucket.
Vendor credits lower what you owe and should be applied to the correct vendor's bill. They are not customer credit memos and do not create cash receipts. An aging report supports payment planning, but confirm disputed bills and available credits before authorizing a payment. A negative vendor balance may represent an overpayment rather than an expense that needs to be posted again.