The following are things I had to do to get vnstat to work. I think it would be useful if the wiki had the info added.
After installation is complete I had to do the following. The clue to me was the warning in red on the SME Server Manager page saying the the SME had to be configured and rebooted or it would be left in an unknown state. I don't know if this happened to me because I did some wrong, but, if not, this seems to be an important missing step.
signal-event post-upgrade
signal-event reboot
Understanding that wikis are supposed to be for SME Server and that the vnstat wiki is supposed to be about getting vnstat installed in SME, it would be useful to have a section in the wiki for trouble shooting or something like that where the different things that people have had to do to get vnstat to work could be dropped in. I and at least one other person (Mntsnow) had to do the following to get it to work.
1. Edit the vnstat cron file.
nano /etc/cron.d/vnstat
2. Change the line
*/5 * * * * nobody /usr/sbin/vnstat.cron
to
*/5 * * * * root /usr/sbin/vnstat.cron
3. Restart crond.
service crond restart