You can uninstall or removes an installed vnstat package itself from Ubuntu 17.04 (Zesty Zapus) through the terminal,
$ sudo apt-get remove vnstat
If you would like to remove vnstat and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove vnstat
If you use with purge options to vnstat package all the configuration and dependent packages will be removed.
$ sudo apt-get purge vnstat
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove vnstat