There are about 3-4m PC's that can run the Vive right now due to the power it requires (970 or higher GPU). When the Kinect came out there were 70m Xbox's. We really can't compare the two. The xbox had 20x the population.

The Vive has a lot against it: the price, the cost of a gaming PC, etc. 100k sales is pretty impressive. I assumed 50k would be a stretch by now.