Install in RHEL

Disclaimer

These instructions assume

the user wants to install using the source package instead of distribution provided packaging
access to 'root' account or user with admin permissions
use only distribution release specific instructions
some basic Linux skills
commands aren't copypasted blindly
that the process is stopped in case of errors

Open a terminal and gain root permissions

su -

Verify that root has been gained

whoami

which should reply "root".

Install needed development packages

yum install make gcc wget sqlite sqlite-devel

optional additional packages for image output support

yum install make gd gd-devel

Get the latest release and extract it

cd /usr/src
wget https://humdi.net/vnstat/vnstat-2.12.tar.gz
tar zxvf vnstat-2.12.tar.gz

Compile and install the binaries

cd vnstat-2.12
./configure --prefix=/usr --sysconfdir=/etc && make all && make install

Install the service files and start the daemon

cp -v examples/systemd/vnstat.service /etc/systemd/system/
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.