postfix+dovecot store maildir format mail in /var/mail

Since you've already configured Dovecot, the easiest solution to your problem is to use deliver from Dovecot as the mail delivery agent (MDA).

This could be done by adding this line to main.cf (paths may vary according to your distribution, on Debian it's /usr/lib/dovecot/dovecot-lda):

mailbox_command = /usr/libexec/dovecot/dovecot-lda -f "$SENDER" -a "$RECIPIENT"

If you got a larger scale installation and want to use virtual users, you've got to setup a service in master.cf accordingly, e.g. like this:

dovecot   unix  -       n       n       -       -       pipe
flags=DRhu user=vmail:vmail argv=/usr/local/libexec/dovecot/dovecot-lda -f ${sender} -d ${recipient}

Of course you shouldn't use those without carefully crafting your own configs.

Further information can be investigated here:

http://wiki2.dovecot.org/LDA/Postfix