Hello @Randy Fleszar - Thank you for reaching out!
For Azure backup agent, you need to delete the protected items from MARS console as well. For instructions refer to this link - https://learn.microsoft.com/en-us/azure/backup/backup-azure-delete-vault#delete-protected-items-on-premises
After deleting protected items in MARS, then delete backup items from portal, that should delete everything related to Azure backup agent. Post this operation, you should be able to delete recovery services vault (RSV) if there are no other backup items in the RSV.
Feel free to reach out to us, if need further assistance in this matter!
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