We’ve had very positive results accelerating XenForo with Varnish (in a cluster configuration). In terms of VCL, we’ve taken a slightly different approach but it results in better caching / acceleration. We’ve allocated a small per-user cache (logged in users), which makes XenForo pages load even faster than normal conditions. This reserved cache is rather transparent to the end user so no code changes are required. In fact, the acceleration we provide requires no modification to any XF files at all.
Static files, however, are stripped out of cookies in the Varnish VCL and served from the same cache area. The performance gains are tremendous. We’ve already published a Varnish plugin for cPanel and DirectAdmin that incorporates these tweaks. Feel free to read more about it here: http://www.unixy.net/varnish