with vnstat 1.12 (as in 14.10 and later) vnstat changes the user id during startup to whatever is defined as DaemonGroup and DaemonUser in /etc/vnstat.conf. Hence, the proper way would be to
(0) upgrade to vnstat 1.12 (if not already done)
(1) change in /etc/vnstat.conf DaemonUser and DaemonGroup from "" to "vnstat", and
(2) in /lib/systemd/system/vnstat.service or /etc/init.d/vnstat depending on start-up system user from "vnstat" to "root"
with vnstat 1.12 (as in 14.10 and later) vnstat changes the user id during startup to whatever is defined as DaemonGroup and DaemonUser in /etc/vnstat.conf. Hence, the proper way would be to system/ vnstat. service or /etc/init.d/vnstat depending on start-up system user from "vnstat" to "root"
(0) upgrade to vnstat 1.12 (if not already done)
(1) change in /etc/vnstat.conf DaemonUser and DaemonGroup from "" to "vnstat", and
(2) in /lib/systemd/