I was helping troubleshooting the 'RPC Server is unavailable' issue this week. The message was shown on a Vista client with VMRCplus connecting to a remote host.

Although I have included this issue in the VMRCplus help file (VMRCplus.chm) I realized that the information may not be enough. It took considerable effort to get VMRCplus working on Vista where the Windows Firewall simply blocked the traffic.

So what was the issue?

Windows Firewall contained an exception for VMRCplus. And the path pointed to the correct location of VMRCplus which, in the default installation, is %ProgramFiles%\microsoft vmrcplus\vmrcplus.exe.
Everything seemed OK but still the RPC message showed.

I deleted the exception and was hoping for Windows Firewall to popup the Unblock/Keep blocking message. But nothing happened except for VMRCplus being persistent with the RPC message. Adding the exception back in did not help either.

The solution in this case was to delete the exception, reboot the client and start VMRCplus again.

At that point, the Windows Firewall popped up the dialog which offers to either unblock or keep blocking VMRCplus. Selecting Unblock still showed the RPC message because it was 'too late' at that point. Connecting again did not succeed either. I had to exit VMRCplus and start it up again.
Now the Windows Firewall allowed VMRCplus through and I could finally connect.

From my experience in troubleshooting this issue it is necessary to have the Windows Firewall exception for VMRCplus in place prior to launching VMRCplus. But if it's there and you still get the RPC message, follow the path as described above.