Open webmin, click webmin, webmin configuration, webmin modules, make sure "local file is checked, click '...' point to vsftpd.tar.gz then goto ok.

You should now see vsftpd in the servers in webmin, however it doesn't work. you need to click on the edit config and change the path to the vsftpd.conf to /etc/vsftpd/vsftpd.conf