These instructions assume
Arch is a rolling release distribution and the vnStat package in the official repository is usually updated within a week from a new vnStat release. As a result, compiling and installing vnStat from the source package isn't usually needed for getting the latest version.
As root, install vnStat from the official repository
pacman -Syu vnstat vnstati
Enable and start the service
systemctl enable vnstat systemctl start vnstat
Verify that daemon process got started
pgrep -c vnstatd
which should reply "1". A reply of "0" indicates that the process has failed to start.