First, open up terminal and login as root:
<code>su - root</code></br>
<i>At this time the commands have to be run in root. Sorry for any inconvinence</i>
<code>vnstat -i eth0 -u</code>
<code>chown vnstat:vnstat /var/lib/vnstat/*eth0</code>
<code>vnstat -i eth0 -u</code>
<code>chown vnstat:vnstat /var/lib/vnstat/*eth0</code>