Vanco gateways only allow refunds to be processed from their own web site.  To keep your software and gateway records in sync, use the Manual Refunds process which does NOT submit the refund to your gateway for processing.  Instead, it simply records the refund transaction as having been processed.

Note:  if you are using a Vanco gateway, your event site should be configured to process refunds manually.  Please contact [email protected] if you need help with your refund settings.