Are you trying to use webmail or do it through the panel? Cause you should be able to do. I use SOGo for the web mail and the vacation mode does allow you to set a date and time for when auto response is enabled and when it's disabled. Likewise, if you go the email account's settings in the panel you can choose when to switch it off (although it's not as specific compared to doing it through SOGo, I haven't tested it in any of the other webmail interfaces but I would think it's the same).