It may be doable, but I doubt it's easy... because to change the vacation message it's required that you're logged in as the main domain user. Which would mean you'd have to write something that would find the domain, find the user, and then the password, and then login as that user to change the message.
Or write something to run as root and make the changes manually, ignoring the Control Panel completely.
Both are potentially insecure; the latter quite a bit more than the former.
Jeff