where can i find varnish log files? /var/log/varnish is empty

by default varnish will not log anywhere.. you have to run a command to get it to show logs

You can run the command

varnishncsa

For more info on how to use that command to write to a log instead of stdout

http://www.go2linux.org/linux/2011/05/configure-varnish-logs-varnishnsca-logrotate-and-awstats-1014