From https://www.vaultproject.io/docs/audit/

  Because every operation with Vault is an API request/response,
  the audit log contains every interaction with Vault, 
  including errors.

  The data...will be hashed with a salt using HMAC-SHA256.

  The purpose of the hash is so that secrets aren't in 
  plaintext within your audit logs. However, you're still
  able to check the value of secrets ...by using the 
  /sys/audit-hash API endpoint

Does this not cover your use-case?