I suggest you follow this tech note so the name does not resolve, it will be explicit.
Also,
Do this to troubleshoot
On the EV server
1. Create an outlook profile on the EV server for the System mailbox of the Sub-domain exchange server logged in as the VSA.
2. Launch Outlook and verify that you can login to the system account under the security context of the VSA.
3. Verify that you can send a message logged in as the VSA on behalf of the sub-domain system account.
4. This will verify that you have "Send As" permissions for the VSA in that sub-domain.
If this fails then AD rights will need to be looked into.
Jim S.