Setting the backend setting for Apache + Nginx
Why not just nginx + varnish?
nginx and apache are both webserver, of course you can use nginx proxy_pass... but if this is the same server I would ditch apache or nginx... choose one.
In any case "Varnish is a web application accelerator. You install it in front of your web application and it will speed it up significantly." (copy paste form the home page at their site)
So I think first you should decide if you want nginx or apache has you're web server.