The auto-responder feature allows dummy email address to be connected to some content, sent whenever a message is received for this address. For example, for a vdomain called vdomain.com one can create a file called /var/spool/vmail/files/vdomain.com/prices.auto and this file will be sent whenever someone send a message to firstname.lastname@example.org. This pseudo user prices does not have to be created in the account database. The file just have to exist. Note that this file has precedence over accounts and aliases.
The vacation is handle using roughly the same logic. For user joe member of the domain vdomain.com, if you create a file /var/spool/vmail/files/vdomain.com/joe.vacation it will be sent to the sender whenever a mail is received. Bulk messages and mail-daemon related are omitted.
Also, a file called /var/spool/vmail/files/vdomain.com/joe.froms is maintained. It is used to record the fact that an email was sent to a sender once, so we won't send the vacation message over and over to the same person.
Please test this and submit comments. Once settle, we will had the administration tool allowing users to change their vacation message themselves.