THIS IS NO LONGER APPLICABLE STARTING FROM cPanel Varnish Plugin v6.0
The second you enable SSL in Varnish all current SSL installations, those you completed via cPanel, will stop working. This is because the plugin reprograms the SSL port of 443, that Apache listens on, to 4443. Nginx is then made to listen on port 443 (Nginx terminates SSL traffic for Varnish).
Please, plan ahead for this change (during low traffic periods) so you have a minute or two to enter the SSL certificate info into the WHM Varnish plugin screen.