+ sub addressing setup

[lists at sbt.net.au]

originally, I've setup all spam marked email to be sent to generic
spam at mailhost.tld

that works, but, everyones spam in one account

I would like to setup spam subaddressing to send amavis picked spam to
spam subfolder for a user to assess, etc

I've setup my own address, with voytek+spam at mailhost.tld, it works fine,
mail gets +spam address, gets to my spam folder

I'm trying to setup some virtual users on virtual domains as:

user at vdom1.com
user+spam at vdom1.com

when I sent an email (from webmail on mail host) to user+spam at vdom1.com;
'.spam' folder was created and test email  was delivered to spam
subfolder, all correct

BUT, none of real spam email ends up in user at vdom

I guess I need something additional for virtual domains...?

the spam marked emails for my virtuals end up in spam@ email address,
EXCEPT voytek's spam

I guess I'm missing virtual sub address...? in postfix...?
(though it seems to work when tested from server webamil)
thanks for any pointers