Well, light is propogating through that vacuum. Otherwise you wouldn't be able to see the feather and ball bearing.
Ok so you are saying that scientists have determined the speed of light by simply observing a near-empty vacuum inside a lit room?
Oh, sorry, I didn't know you wanted how they found the speed of light.
Well, back in the day some fellow noticed Jupiter's moons' periods varied. That is, sometimes they seemed to go faster, others slower. Takeing careful measurements, he figured that light moved about 220,000 km/s.
Slightly more recently astronomers used telescopes and the apparent positions of stars. They noticed the stars seemed to wobble a bit. One fellow measured the wobble and calculated that light moves about 10,000 times faster than the Earth.
In modern times, we regularly measure the delay between various spacecraft. With so many robotic probes, satellites, etc. out in space, we can measure whatever distance we want as often as we want.
One can also set up a cavity resonator, in which a light of a certain wavelength will resonate. If you know the size of the container and the wavelength, you can calculate the speed of light. This is easy to do in a small vacuum.