Set Postfix and Dovecot mail server to store emails as 'vmail:vmail' user group

This is the way I configured it and it works for me:

On postfix master.cf file, add at the end:

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

On dovecot/conf.d/10-master.conf configure service auth on the following way:

service auth {
  unix_listener auth-userdb {
    mode = 0666
    user = vmail
    group = vmail
  }

Hope it works for you. Regards