1. https://community.ubnt.com/t5/EdgeMAX-CLI-Basics-Knowledge/EdgeMAX-Add-other-Debian-packages-to-Edge...

2. sudo apt-get update

3. sudo apt-get install vnstat vnstati

4. vnstat -u -i eth0 ( or any other interface you need )

5. /etc/init.d/vnstat start


To ensure vnstat starts on boot run this on the command line:
 
    ln -s /etc/init.d/vnstat /config/scripts/post-config.d/vnstat

Add the following to /config/scripts/post-config.d/vnstat:

    if [ ! -d /var/log/vnstat ]; then mkdir -p /var/log/vnstat; cp /config/scripts/vnmon/vnstat-data/vnstat/* -R /var/log/vnstat; fi