All discussions are tautology without understanding what Vacuum is.
The problem is, people don't listen explanations of others, because they prefer to think independently under hope, they'll find proper solution first. In such way, even though such explanation is proposed already, people repeating their own ideas like mantra. There is quite thin boundary between creative (i.e. positive) and ignorant (i.e. negative) stance: if you're speaking, you cannot listen and vice-versa.
In general, SRT can be explained without exact knowledge of true nature of vacuum at all. What we know, light is wave of vacuum environment analogous to wave at water surface. Now we can put a question: can we see water surface by using of water wave?
The answer is: no, by using of water wave is impossible to see water surface, because this surface is serving like environment for spreading of water wave instead.
After then another question can be raised: Can we see the motion of water surface by using of surface waves?
The answer is: no, if we cannot see water surface by using of surface waves, then we cannot detect its motion by using of these waves as well. It means, EVERY wave is invariant to (motion/reference frame) of its own environment by its very definition. And this is basically, what the postulate of light speed invariance of special relativity theory is saying. The rest of SRT is just a thin mathematical envelope surrounding this basic concept.
Therefore I say the SRT is theory about Light Quanta.
Sorry, SRT doesn't care about Light Quanta at all. On the contrary: SRT is just describing light as a flat wave without any particles at all.
Existence of light quanta follows from particle nature of light environment and its relevant only for light waves, whose wavelengths differ from wavelength of cosmic microwave background. Light waves of shorter wavelengths are propagating through such chaotic field like photons, whereas waves of larger wavelengths are propagating through such field like tachyons, i.e. gravitational waves.
