To run vnstat from cron, add the following line to your crontab:

  0-55/5 * * * * if [[ -x /usr/bin/vnstat && $(ls /var/lib/vnstat/ | wc -l) -ge 1 ]]; then /usr/bin/vnstat -u; fi

To run vnstat from /etc/rc.d:
  chmod +x /etc/rc.d/rc.vnstat

Note that to have vnstat start automatically this way it should be
called from rc.local (and optionally rc.local_shutdown) like so:

  In rc.local:
  # Start vnstat
  if [ -x /etc/rc.d/rc.vnstat ]; then
    /etc/rc.d/rc.vnstat start
  fi

  In rc.local_shutdown:
  # Stop vnstat
  if [ -x /etc/rc.d/rc.vnstat ]; then
    /etc/rc.d/rc.vnstat stop
  fi