We can't produce a true vacuum, so how do we know how fast light travels through 1?

One reason is this: at low pressures, the electromagnetic properties (electric permittivity ε

_{0} and magnetic permeability μ

_{0}) of air (for example) vary very little with pressure, so making the extrapolation to complete void gives the same result. You know that light's speed is related to these properties:

c = 1/Sqrt(μ

_{0}ε

_{0})