It's been announced that the HTC Vive is going to get infrared eye tracking as an add on. The company behind this is 7invensun, who were recently accepted onto the HTC Vive X Accelerator program. Called "aGlass" it will use the spare USB port which is hidden under the headset's cable retainer (I wondered why HTC put that there, but now we're beginning to see why!). It tracks the user's eye movements using a ring of IR LED's and an inbuilt camera, and sends the info back to the PC rendering the content. One of it's uses will be to enhance the graphics capabilities (called foveated rendering).
One other feature of the 7invensun kit is the ability to insert corrective lenses. This means for those who usually have to wear glasses to play the Vive could ditch them altogether by inserting the corrective lenses needed directly into the headset. Of course, this isn't helpful if the headset is shared between several players who need varying level of corrective vision. That said, from the screenshots on their website, it appears to show that the lenses can be flipped up into the roof of the HMD.
Currently the aGlass is only going to be available as a development kit in China, but considering the potential of the device, I'm sure it will be available for other countries within a few months.