An autoresponder / vacation message is not for a webmail client. It could be done, but you'd have to make it so configurable so that every IMAP server is taken into account because all handle them a little differently (much like SQL code). The same type of conversation is happening with filters and whether they should be included. They can be, but it has to be really configurable.
The issue lies in the fact that if you end your webmail session, the autoresponder and vaction messages won't work (if they're client-side) because there's no client to respond.... but if you set it up server-side, it works until you turn it off; however, every server sets these things up differently, so this could be a huge undertaking to get all the workings correct.
