Why does Cyrus set the MAIL FROM address of the sender of vacation responses to '<>'?
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Because many of the Sieve features are autoresponders, Cyrus tries to
be a good network citizen by adhering to the appropriate RFCs:

From **RFC 2821**:

   This notification message must be from the SMTP server at the relay host
   or the host that first determines that delivery cannot be accomplished.
   Of course, SMTP servers MUST NOT send notification messages about
   problems transporting notification messages. One way to prevent loops in
   error reporting is to specify a null reverse-path in the MAIL command of
   a notification message. When such a message is transmitted the
   reverse-path MUST be set to null (see section 1.5.5 for additional
   discussion).

A MAIL command with a null reverse-path appears as follows:

   "MAIL FROM:<>"

Also from draft-moore-auto-email-response-02.txt:

   The primary purpose of the MAIL FROM address is to serve as the
   destination for delivery status messages and other automatic responses.
   Since in most cases it is not appropriate to respond to an automatic
   response, and the responder is not interested in delivery status
   messages, a MAIL FROM address of <> MAY be used for this purpose.

This is of course slightly more applicable to reject than it is to
vacation, but there isn't a very strong argument why they should be
treated differently.

Note that draft-moore-auto-email-response-02.txt also states:

   A MAIL FROM address which is specifically chosen for the purpose of
   sending automatic responses, and which will not automatically respond to
   any message sent to it, MAY be used instead of <>.

Therefore, it may be reasonable to have this be a configurable fixed
address in the future.