so for jessy's dovecot :

/etc/dovecot/conf.d/10-master.conf

service auth {
    # Postfix smtp-auth
      unix_listener /var/spool/postfix/private/auth {
        mode = 0666
      user = postfix
      group = postfix
    }

/etc/dovecot/conf.d/10-auth.conf

    #!include auth-deny.conf.ext
    #!include auth-master.conf.ext
    #!include auth-system.conf.ext
    #!include auth-sql.conf.ext
    #!include auth-ldap.conf.ext
    !include auth-passwdfile.conf.ext
    #!include auth-checkpassword.conf.ext
    #!include auth-vpopmail.conf.ext
    #!include auth-static.conf.ext

/etc/dovecot/conf.d/auth-passwdfile.conf.ext
    passdb {
      driver = passwd-file
      args = scheme=CRYPT username_format=%u /etc/dovecot/users
    }

    userdb {
      driver = passwd-file
      args = username_format=%u /etc/dovecot/users
    }

/etc/dovecot/conf.d/10-mail.conf
mail_location = maildir:/var/vmail/%d/%n/