How Vacation autoreply can be done in new version 0.24?
I believe that can be set in the user's settings in roundcube - check box.domain.com/mail, login and see settings.
EDIT: Nevermind, cannot find the setting in there myself..
You should be able to create a mail filter rule in Settings -> Filters that has the same effect, i.e. reply to every email with a message. (This is how it was done under the hood by the old plugin.) Something like this: