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The speed of light in a medium is related to the refraction it undergoes per Snell's law and the speed is inversely proportional to the index of refraction. It is not internal micro- or pico-reflections but the quality of the medium to sustain the wave characteristics of light that slows light down. The slowing of light to a foot a second in Bose-Einstein concentrate is due to the index of refraction of the medium being very high. The index of refraction of a pure vacuum is 1.0000 . . . by definition. If the index of refraction is negative then the light would go backwards. Of course, what the ultimate reality might be probably can't even be guessed.

I thought the angle of reflection always equaled the angle of incidence. If the refraction index is negative, the angle of reflection would be less than the angle of incidence and if the refraction index is good enough, could actually equal the angle of incidence. In that case, the refractive material would be the perfect reflector and if standing in front of one, you would see yourself regardless of your viewing angle.