You can check, at the online Doc, about the Virtual Service Execution Mode:
Execution Mode - DevTest Solutions - 10.0 - CA Technologies Documentation
There's a mode called Stand In, that the virtual service will foward calls to the real service (live system) when a match is not found
- Stand In
The virtual service environment (VSE) responds like it does in Most Efficient mode, except that when it normally sends back an unknown response, it forwards the request to the live system instead.