Hello John,
Varnish issn’t currently supported with in Hestia. If you still want to add it you will have to look up some tutorials on the internet.
Personally I use Nginx “proxy” cache for it an my experience with it are good. How ever if you have Nginx installed as stand alone it will not help you. I am currently working on a pull request / feature to add fast cgi cache support for Nginx standalone to Hestia. How ever it will take more then a few days…
It is currently planned for 1.3.2