Vacuum 

I have made some changes in wording that I feel more comfortable with. The modern vacuum is not the same as the classical vacuum, even in principle, and it seems to me that some distance should be placed between the various vacuums: the now archaic "vacuum" of classical electrodynamics, the BIPM official "vacuum", the realizable vacuum of interstellar space, and theoretical vacuums like the QCD vacuum. 

Another issue is the introduction by BIPM of the "defined value" of c. This definition is tantamount to a definition of BIPM "vacuum", and is a bit unsatisfactory as there is no theoretical definition of this "vacuum". Rather, it is a moving target that is actually defined in an empirical manner through a library of BIPM corrections that are not completely specified but left up to "standard best practices". About all one can say in its favor is that it is a "reduction to practice" of "vacuum". However, having no theoretical basis outside of this (ever changing) operational definition, it is subject to definition by committee based upon criteria some of which have no physical basis, but are based upon (ultimately) convenience, politics, and economics.