I am currently dealing with this problem but a little further along. I have chosen to use the same domain it will be easier for in long run. The VCS has loop detection and you can push the priority to push calls to CUCM with that domain.
Or you can built a SIP truck and set priorities for both.
http://www.cisco.com/c/dam/en/us/td/docs/telepresence/infrastructure/vcs/config_guide/X8-5/Cisco-VCS-SIP-Trunk-to-Unified-CM-Deployment-Guide-CUCM-8-9-10-and-X8-5.pdf