The Exchange database must be in a Clean Shutdown state to mount the databases after going into Live mode. Restores to the VFC are successful regardless of whether the databases are in a Clean Shutdown state, but you cannot access Exchange databases if they are in a Dirty Shutdown state.
If you cannot mount Exchange databases after going into Live mode, see this Microsoft article to determine whether the databases are in a Dirty Shutdown state: Exchange Database is in a Dirty Shutdown State.