The speed of light depends on the medium, but in vacuum it's always the C. About the question of density, when we are dealing with expansion of universe, we should consider that the fabric of the cosmos itself is expanding, hence all the galaxies and galaxy clusters get drifted apart from each other (expect for gravitationaly bound objects which are attracted to each other more than what expansion of cosmos can drift them apart).

Therefore the density of universe also changes, but it doesn't mean that vacuum has changed or anything, merely it means that there are more vast vacuum space in the cosmos now.

Also consider if a beam of light, for example has spent a million years to travel from one galaxy to another, it would have covered one million light years, but with the expansion of universe during that time, the distance between the two galaxies are about for example 1.1 million light years (numbers are arbitrary) during this time the speed of light hasn't changed at all, it is always C, but the meters that the light has traversed are expanded (or squeezed if the expansion of cosmos is reversed), the speed hasn't changed, but the meters got inflation (kinda like my salary being constant and expenses getting inflation 🙄🙄) this notion is called comoving and proper distance. I hope I could have covered your question.