An EM particle-wave propagates in a vacuum at a constant speed $c$, independent of the source/receiver but dependent on most mediums it moves through. Since the vacuum is a medium, and if logically permissible, all mediums affect the speed of light, these affects of a medium can:
alter the speed of light.
possibly alter the distance light has to travel between source/receiver.
subsequently alter the interval of time necessary to travel that distance.
Therefore the vacuum will affect the speed of light, and the speed light is measured to move at will depend on its medium, the vacuum. If each medium has a refractive index measuring this affect on light speed in a medium, then in a vacuum it would be at $c$. Is it possible that light has a true speed beyond what we observe in a vacuum? Also, since light travels the shortest distance/in the shortest possible interval of time through any medium, despite changes of lights speed in that medium,: does the speed of light actually slow down in a medium or just appear to slow down because of path deviations caused by a medium that alter distances thereby altering time intervals?