That is exactly what the aether was conceptualized to cover. A media through which EM waves travel.

The M-M experiments experimentally demonstrated that the concept was not needed.

One may state that based upon quantum mechanical considerations that a classical vacuum is an impossibility.

Normally, however the term vacuum is intended to convey the absence of all.

The purpose of my original post was to clarify what is the actual speed-limit of light and that it is defined in vacuo, which has been experimentally verified until, perhaps this recent observation.