Thanks for the guide!
I updated the /etc/vnstat.conf as follows:
DatabaseDir "/var/log/vnstat"
Then I try restarting the service:
sudo service vnstat restart
I get the following response:
[ ok ] Stopping vnStat daemon: vnstatd. [....] Starting vnStat daemon: vnstatdError: Unable to open database directory "/var/log/vnstat". Make sure it exists and is at least read enabled for current user. Exiting... failed!
So I made the directory and now the service starts up fine.
But I'm worried that since I'm using tmpfs, on reboot it'll fail again? Am I missing something?