Set the Vim filetype for sieve files.

dovecot/before.sieve

+# vim ft=sieve
+
 require ["fileinto", "envelope"];
 if anyof (header :contains "X-Spam-Status" "Yes",
           header :contains "X-Virus-Status" "infected")

dovecot/nimrod.sieve

+# vim ft=sieve
+
 require ["fileinto", "envelope", "imap4flags"];
 if address :is "from" "nimrod@shore.co.il" { setflag "\\seen"; }