[Exim] autoreply using vacation program

We need to use the vacation program for auto-reply for staff. In our
current set up it is widely used by staff. However in our future set up the
staff e-mail will not be on the mail-hub.
In the Exim FAQ, there is an explanation of vacation program on mail hub
(Q0431). It works fine only if the staff were on the mail hub!
The other example (Q9813), when there is no local user points to example
C019. If I understand correctly, it will be necessary to re-load the
configuration file each time a user goes to holiday or come back from
holiday. Is there any way of getting round this problem and use the
vacation program on staff server?
The staff server is also running Exim except all mail is routed through the
mail hub using the following configuration in the ROUTERS section:

smart_route:
driver = domainlist
transport = remote_smtp
route_list = "* mailhub.unl.ac.uk bydns_a"

Sujit Choudhury
University of North London