If the vacuum were "emptiness" it were easy to accept that there is no "preferred frame of coordinates" and the light velocity is the same in any inertial frame of coordinates. But the vacuum is an ocean of "virtual" particles (that carry energy with themselves, and quickly can disappear). Essentially, the vacuum is not empty.
Now, all these particles don't provide, at least in some regions of the space, a PREFERRED frame of coordinates? Can't we talk of a frame AT REST with respect to these particles and frames IN MOVEMENT with respect to them? If, suppose, we can say that there is a frame at rest with respect to these particles, why doesn't the light velocity differ in this frame from its value in other frames? Isn't the light scattered by these particles? A beam prepared as monochromatic, won't suffer a dispersion in its wavelength?
To be clear, the principle of the relativity that says that the light velocity is the same in any inertial frame, seems to me questionable vis-à-vis possible interactions of a beam of light with such particles.