For those of you wondering btw, the vive face tracker isn't really a vive anything, it's a tobii eye tracker with two of the cameras cut off. 

A tobii tracker has two big IR diodes and four IR cameras, one placed on each end of the device and two placed next to eachother in the middle. The two on the ends are used for tracking IR reflections from your retinas, the two in the middle are used more normally, mostly for things like "Microsoft Hello". The Vive FT is just the IR diodes and the two "Hello" cameras.
It even installs Tobii software on your computer.