No, I'm saying VDN 1234 can either route to 5678 and follow 5678's coverage.
Or, VDN 1234 can queue to a hunt of which 5678 is a member, and if unanswered, go to 1234's mailbox
Or, VDN 1234 can route to 5677, of which a bridged appearance lives on 5678's set and if unanswered, go to 5677's mailbox. Or have 5677 be a secondary mailbox of 1234, or have 5677's coverage path go to a vdn that routes to "messaging split 99 for 1234"
Or, 1234 can be a X ported station with a bridged appearance on 5678.
All to say, you need an extra step in between routing from the VDN to 5678 and getting to the mailbox for 1234 and having 5678's direct calls go to 5678's mailbox.