The VisionPrescription resource has a very low maturity rating. It was developed by the financial community and we've been waiting for some domain experts to review it. For now, you will need to use extensions for any needed data elements it doesn't provide, but feedback on what's actually needed in the vision community would be appreciated. (We're evaluating deprecating the resource in R4 and replacing it with a profile on the revised DeviceRequest resource which supports providing multiple parameter values (the explicit vision parameters would become name-value pairs with codes for things like eye, sphere, cylinder, etc.). You can provide feedback by using the "propose a change" link at the bottom of any page in the spec. You'll need to register the first time, but that's free.