[Exim] vacation condition broken

I like exim's built-in vacation reply feature, but it doesn't seem to be
working as advertised. Well, it's either the ${if {}{}{}} construct, or
the 'condition =', or something else that isn't mentioned in any
examples I've seen in mailing lists and web pages I've searched.

I've been told to *not* reply with vacation messages to messages that
are mail-bounce errors (like those caused by NetSky or MyDoom), nor
messages delivered to group aliases (/etc/aliases:"team: bob, ted,
alice"), nor mailing lists nor messages tagged as spam.

I have this configuration, which I think should work, but I get a
vacation autoreply for every message despite what the "condition =" is
set to. Since it depends on headers and variables like $sender_address,
I can't just feed it into exim -be. Could someone have a look and tell
me what I'm doing wrong?

---begin quote
### Started with http://www.afp548.com/Articles/Jaguar/eximvacation.html
user_vacation:
  driver = smartuser
  no_expn
  no_verify
  unseen
  require_files = /var/www/vacation/user/${local_part}/vacation.msg
  # do not reply to errors or SPAM-tagged messages or alias lists
  # or junkmail
  condition = " \
    ${if or { \
              { eq {$sender_address}{} } \
              { def:header_x-spam-flag: } \
              { !match {$header_to:}{(?i)$local_part} } \
              { match {$h_precedence:}{(?i)junk|bulk|list} } \
            }{no}{yes} \
     } "
  #condition = "${if or {{match {$h_precedence:} {(?i)junk|bulk|list}} \
  # {eq {$sender_address} {}}} {no} {yes}}"
  # do not reply to errors and bounces or lists
  senders = " ! ^.*-request@.*: \
              ! ^owner-.*@.*: \
              ! ^postmaster@.*: \
              ! ^listmaster@.*: \
              ! ^mailer-daemon@.*: \
              ! ^root@.*"
  user = ${local_part}
  transport = vacation_reply

---end quote