The very idea of using the 1887 experiment from Stone Age to defend GR seems ridiculous. I understand why you are doing that: there are no modern experiments, which you can use to defend your point, but still. Moreover, interpretation of Michelson-Morley experiment as «it proves that physical vacuum does not exist» is frivolous. Let me quote Wikipedia:
> The Michelson–Morley experiment of 1887 had suggested that the hypothetical luminiferous aether, if it existed, was completely dragged by the Earth. To test this hypothesis, Oliver Lodge in 1897 proposed that a giant ring interferometer be constructed to measure the rotation of the Earth; a similar suggestion was made by Albert Abraham Michelson in 1904.
> the first interferometry experiment aimed at observing the correlation of angular velocity and phase-shift was performed by the French scientist Georges Sagnac in 1913. Its purpose was to detect "the effect of the relative motion of the ether".
> In 1926, an ambitious ring interferometry experiment was set up by Albert Michelson and Henry Gale. The aim was to find out whether the rotation of the Earth has an effect on the propagation of light in the vicinity of the Earth. The Michelson–Gale–Pearson experiment was a very large ring interferometer, (a perimeter of 1.9 kilometer), large enough to detect the angular velocity of the Earth. The outcome of the experiment was that the angular velocity of the Earth as measured by astronomy was confirmed to within measuring accuracy. The ring interferometer of the Michelson–Gale experiment was not calibrated by comparison with an outside reference (which was not possible, because the setup was fixed to the Earth). From its design it could be deduced where the central interference fringe ought to be if there would be zero shift. The measured shift was 230 parts in 1000, with an accuracy of 5 parts in 1000. The predicted shift was 237 parts in 1000.
So, Michelson-Morley experiment confirmed that physical vacuum is dragged with Earth, and then Michelson–Gale–Pearson experiment confirmed that it is not completely dragged by Earth. It far from «physical vacuum doesn't exist» mantra from «shutup and calculate» guys.
In this century,
* the existence of Higgs boson, which gives mass, and Higgs field, which exists at every point of our Universe, was proved in 2012;
* the existence of gravitational waves in physical vacuum is proved by LIGO/Virgo interferometers.
Thus, today we have Higgs field, which is presented at every point, which can be used as 0 point, but we cannot detect it. However, we can use CMB (light from distant galaxies with red shift z=1000) as 0 point.
IMHO, relativity is not necessary at this point.