The Varnish documentation in the cookbook isn't 100% correct. In order to purge you need to make sure the PURGE method is dealt with in vcl_recv, otherwise Varnish is going to fallback on default behaviour.

This means Varnish will consider it a non-RFC-compliant method, will pipe the request to the backend and ignore the Varnish caching mechanisms.

To avoid that simply catch PURGE requests and immediately perform a lookup to the cache. By doing this you'll skip default behaviour.

It's just a matter of adding the following piece of VCL

sub vcl_recv {
    if (req.request == "PURGE") {
        return(lookup);
    }
}

A full example of an acl would also be interesting.

I'm going to create a pull request and link to this issue as a reference.