Apple has the prescription lenses in the stores. If you bring your glasses, they have a device that will shine some lights through your glasses to determine your prescription and pop out the correct lenses needed for the Vision demo.
The intake form asks for certain things (like prism vision) and it also asks if you know your prescription ahead of time. I can't imagine that they'll have all of those specific fixes but, for the majority of people, you can simply go in and get the right prescriptions lenses. They even email you a link to get your specific prescription of lenses after the demo.
