Yes exactly there out of office/vacation messages though the admin area. In the past I have used alternatives like ISPConfig which allows you to administar each vacation message from within each users settings however this has meant having to select each account individually, still through one admin interface however.
Heart Internet displays the list of email addresses and then allows you to activate the out of office and gives you a link to edit the message which I have attached with email addresses removed.
I think if on the user list page there was a flag to show if the out of office reply was on or off and could be toggled from this page. Then under the users account, maybe on the general you can edit the out of office responer as an additional text area.
That would be pretty cool.