On CentOS 5.4, the vnstat cron job generates an error:
Error: Unable create database backup "/var/lib/vnstat/.venet0".
By default, vnstat cron job runs as “nobody”, yet the database file “/var/lib/vnstat/venet0″ was owned by root, changing its ownership to “nobody”, fixed the problem:
chown nobody:nobody /var/lib/vnstat/*