The most obvious things to look at first are firewalls, router access control lists, IDS/IPS or any other packet filtering software or hardware between the Orion server and the VMware guests that might be blocking the RPC protocol. The next thing to validate are the credentials. Can you map a drive from the Orion server to any of those VMware guests using those credentials?
Lastly, it may be a group policy setting that's disabling RPC or the Remote Registry Service. Verify the Remote Registry service is enabled and running on those hosts as it's required for Windows Performance Counters. If it's running already, try restarting the service to see if that resolves the issue.