I only mentioned inetd and sshd is examples of daemons that can be enabled/disabled/started/stopped, so maybe vnstatd can be found in the same settings. But having read the manpages for vnstat and vnstatd I'm not even sure you need the daemon.

Permissions? Do you need su for vnstat -w? I suspect it's something simple like that.

I'll take a look at vnstat in Debian when I get home - that might be similar.