Soooo. Speed of light may not be constant in a vacuum... that is not really a vacuum because virtual particles interact with the photon causing it to not go in a straight line.
Did I get that right?
A true vacuum can't exist because Nothing would have at least the single property of absence of something which is not nothing and immediately decay into virtual particles, queue Big Bang. So the point is moot anyway.
Also the photon is not slowed down, it is just delayed or distracted, basically zigzagging around instead of going straight from a to b but still doing so at constant light speed.
Or did those scientist say something different?