Varnish configuration to only cache for non-logged in users

I have a Ruby on Rails application fronted by varnish+nginx. As most of the sites content is static unless you are a logged in user, I want to cache the site heavily with varnish when a user is logged out but only to cache static assets when they are logged in.

When a user is logged in they will have the cookie 'user_credentials' present in their Cookie: header, in addition I need to skip caching on /login and /sessions in order that a user can get their 'user_credentials' cookie in the first place.

Rails by default does not set a cache friendly Cache-control header, but my application sets a "public,s-max-age=60" header when a user is not logged in. Nginx is set to return 'far future' expires headers for all static assets.

The configuration I have at the moment is totally bypassing the cache for everything when logged in, including static assets — and is returning cache MISS for everything when logged out. I've spent hours going around in circles and here is my current default.vcl

    director rails_director round-robin {
  { 
    .backend = { 
      .host = "xxx.xxx.xxx.xxx"; 
      .port = "http";
      .probe = {
        .url = "/lbcheck/lbuptest";
        .timeout = 0.3 s;
        .window = 8;
        .threshold = 3;
      }
    } 
  }
}

sub vcl_recv {

  if (req.url ~ "^/login") {
    pipe;
  }

  if (req.url ~ "^/sessions") {
    pipe;
  }
  # The regex used here matches the standard rails cache buster urls
  # e.g. /images/an-image.png?1234567
  if (req.url ~ "\.(css|js|jpg|jpeg|gif|ico|png)\??\d*$") {
    unset req.http.cookie;
    lookup;
  } else {
    if (req.http.cookie ~ "user_credentials") {
      pipe;
    }
  }

  # Only cache GET and HEAD requests
  if (req.request != "GET" && req.request != "HEAD") {
    pipe;
  }

}

sub vcl_fetch {

  if (req.url ~ "^/login") {
    pass;
  }

  if (req.url ~ "^/sessions") {
    pass;
  }

  if (req.http.cookie ~ "user_credentials") {
    pass;
  } else {
    unset req.http.Set-Cookie;
  }

  # cache CSS and JS files
  if (req.url ~ "\.(css|js|jpg|jpeg|gif|ico|png)\??\d*$") {
    unset req.http.Set-Cookie;
  } 

  if (obj.status >=400 && obj.status <500) {
    error 404 "File not found";
  }

  if (obj.status >=500 && obj.status <600) {
    error 503 "File is Temporarily Unavailable";
  }

}

sub vcl_deliver {
  if (obj.hits > 0) {
          set resp.http.X-Cache = "HIT";
  } else {
          set resp.http.X-Cache = "MISS";
  }
}