I have same problem but I found workaround and it works fine!
You need to edit one line in the /etc/rc.d/init.d/vnstat
it will work fine:
In the line where it says daemon --user vnstat $VNSTATD_BIN -d
will have to change --user "vnstat" into "root"
You can do this manually or execute this command it will do all this by itself [amazing]:
sudo sed -i 's/daemon --user vnstat/daemon --user root/' /etc/rc.d/init.d/vnstat
it should look like this:
daemon --user root $VNSTATD_BIN -d
After all this done then:
sudo service vnstat restart
This works for me nice. I'm not sure if this will work for you. But try. Its worth... good day