John,
Assumption of 'nothing' ends in paradox. Starting from a 'ground state' which acts as a valid datum has no theoretical problem. The problem imagined as an 'add on' to SR causing all the problems, is simply removed by letting these datums be spatially limited and move, which is exactly what we find in nature.
Something is in the 'absolute' vacuum, with known qualities, including the 2.7 degrees! Even current theory can't work without 'dark energy' and 'dark matter'. If you don't like those names think of another i.e. The 'condensate'. Assigning that a 'state of motion', which is what present doctrine won't do, is the only way to resolve all the theoretical issues faced.
A wave on the surface of the ocean is not all 'up', it's half up and half down. The ground state is 'flat', but it cannot be 'zero'! Why would you want it to be?
Peter