[baudrunner]

An example of relative frames of reference are a gas under extreme pressure vs. the atmosphere. The atmosphere is a vacuum compared to the gas under extreme pressure, and when the pressurized gas is released the atmosphere will behave like a vacuum relative to it.

Einstein was not referring to the vacuum of space vs. the gravitational field when stating "We have the laws, but are not aware what the body of reference system they belong to, and all our physical construction appears erected on sand". He was referring to the Universe. At the time he thought that the Universe was static, which it is not, and his theory no doubt influenced the thinking that led to that statement. He was insinuating that a different Universe may have evolved than the one that we are in and then the laws would be different within it, as if suggesting that we are but one possibility out of a possibly infinite realm of possibilities. He was just talking off the top of his head, because he also once said that "God does not play dice". He was also covering his tracks, just in case his theories fell apart, which some did, as in 1929 when Edwin Hubble proved through observation that the Universe is expanding.

There is nothing special about the vacuum of space. It is just unoccupied territory.