For those who have a unix mailserver with procmail-based vacation scripts AND users who do not feel comfortable editing .procmailrc AND you do not use any webmail software (that comes with a vacation plugin) this might solve your problem.
This is Ruby on Rails app that uses SFTP to set up/edit/remove vacation messages to/from mailserver and is backed by (My)SQL database where users are authenticated from. Auth is done by RESTful authentication and the database should be compliant.
This app is localized to English and Estonian, but translation is easy and (extremely) short, so new languages can be added easily.
image © leonardog