SVN r1134 (and lots of trunk revisions before) support start and end dates for vacation.
The XMLRPC interface does not have support for vacation start and end date. Besides the missing feature, this might also lead to interesting[tm] behaviour.
Adding start and end date means we have to change the interface (maybe we can use optional parameters).
While on it, we should also include a parameter for the reply options (once / every hour etc.) - the patch for vacation.pl is somewhere[tm] in the patches tracker already. (We might need another function where the XMLRPC client can get a list of valid options.)
I've just uploaded two patches that should help. There are updates to the squirrelmail plugin that are needed to take advantage of the new function calls. Have it working pretty well. Using http://www.ama3.com/anytime/ to allow the picking of the start and end dates/times. Will probably submit another couple of patches (one being to the VacationHandler) to remove the hard-coded 00:00:00 -- 23:59:25 setting.
Christian - vacation.pl has already been patched for once/hourly/etc intervals... assuming you're referring to the patch from Jan Kuis (hopefully I can spell).
Optional parameters... hmm. I can't remember what Zend_XmlRpc thinks about them; I assume they're allowable.
Fixed since (at least) SVN r1603. Closing as fixed (unless someone complains that xmlrpc is broken by that change ;-)