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

Open a terminal and gain root permissions

su -

Verify that root has been gained

whoami

which should reply "root".

Install needed development packages

zypper install make gcc wget tar

Get the latest release and extract it

cd /usr/src
wget http://humdi.net/vnstat/vnstat-1.15.tar.gz
tar zxvf vnstat-1.15.tar.gz

Compile and install the binaries

cd vnstat-1.15
make && 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.