If you have a trust between the domains, things should work. I know we have users from multiple domains accessing VC resources. However, we grant permission assignments using Local groups on the VC server. So, if you can add your X_domain_user to Y_domain_member_server_Group_Z, you should be golden.
Edit: That said, when I pull up our other domains in VC, the list is populated with users/groups. This is not the case for you. Perhaps our standard way of assigning permissions will still work for you?
