Why convert/alias a@aaa.com to b@bbb.local? Why not setup dovecot to have a mailbox and receive mail for a@aaa.com, allowing sieve to run for a@aaa.com, and users login as a@aaa.com on IMAP to check their mail?
That would work if users only had one alias, but they don't - they have several aliases.
Just to make sure that the vacation plugin works at all I set up a new user with just one mail address x@aaa.com. Vacation replies get sent as long "sieve_vacation_send_from_recipient = yes" is configured in dovecot.conf (my postfix uses different relay hosts).
As said I can't just change users' mail addresses to external ones as they are using several aliases. And I would prefer to use "sieve_vacation_send_from_recipient = no" (due to bounces). I wonder if there is a way to tell postfix to choose a relay host for the vacation replies based on the From: address (which is set by the sieve vacation plugin) rather than the "mail from" address.
-- Cheers spi