[cheald]

Off the top of my head:

    sub vcl_recv {
      set req.http.X-Custom-Cookie-Value = regsub(req.http.cookie, ".*;|^)YOUR_COOKIE_NAME=([^;]+)(;.*|$)", "\2");
    }

Then you can just:

    sub vcl_hash {
      set req.hash += req.http.X-Custom-Cookie-Value;
    }

And now you're cache segmented on that cookie value.