These instructions assume

1. access to 'root' account or user with admin permissions
2. use only distribution version specific instructions
3. some basic Linux skills
4. commands aren't copypasted blindly
5. that the process is stopped in case of errors

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

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.