[Dovecot] Authenting in Linuxconf format

I have an existing email system and I'm trying to addapt dovecot into it. It uses the linuxconf virtual email format. The way it is set up is there there are multiple passwd/shadow files - one for eact domain. They are in standard format. The names of these files are:

/etc/vmail/passwd.domain.com /etc/vmail/shadow.domain.com

I tried this:

auth_userdb = passwd-file /etc/vmail/passwd.%d auth_passdb = passwd-file /etc/vmail/shadow.%d

but - it didn't expand the %d

So - is this going to work? If not - how do you do virtual domains? The documentation is less than complete on this.