While I'd appreciate Plesk adding varnish so that I'd only need to add the vcl, I doubt Plesk will do this, because it would be very support intensive, as the problem with varnish is that it is necessary to configure its vcl to the needs of the backend, and while Plesk supports some backends via the application catalog that they would need to provide vcls for, it gets ugly if you have more than one backend on your domain because the vcls need to be combined which is not always possible, and they don't even have e.g. shopware in there.