Processes don’t work quite like we want with DNS. It’s part of why we haven’t “officially” launched this feature yet! Right now, all processes in the same app show on <app>.internal. It is difficult to run different VMs that expose different services with this setup.
It’s usually better to run caches as separate apps in Fly. You can still use DNS between apps, but you can target things more specifically.
VCL can only use hostnames that return one single IP. I’m not sure how well that will work with Fly hosted apps, especially if you scale to two instances.