This was my first thought too. You can vary by any subset of cookies in Varnish if you simply normalize the Cookie header by stripping out irrelevant cookies. A

This was my first thought too. You can vary by any subset of cookies in Varnish if you simply normalize the Cookie header by stripping out irrelevant cookies. Another trick I've used is to parse the cookie to look for the user ID and set a User-Id header on the backend response and then set Vary: User-Id.