I got the following information from electrical engineer and researcher George Hathaway of TorontoCanada. Dear Mr. Baxter, The speed of light is not a true constant, because it varies with the vacuum energy content (positive, negative, or zero) as you surmise. Photons are affected by propagating through vacuum fluctuations (ZPF). In free space, these fluctuations act to limit the speed of light to what we know it to be via lab experiments. QED theory does not address the variability of permeability and permittivity in undisturbed free-space vacuum because QED is only concerned with the interactions between photons and charged particles. However, the paper cited below addresses the fermionic vacuum ZPF which has been shown to change the permeability and permittivity and thus the speed of light: 1. Marcel Urban, François Couchot, Xavier Sarazin, Arache Djannati-Atai. The quantum vacuum as the origin of the speed of light. The European Physical Journal D, 2013; 67 (3) DOI: 10.1140/epjd/e2013-30578-7