One postulate says:
In vacuum the speed of quantum of light is constant.
It is correct that ‘a vacuum doesn't have a measurable
rest frame’. Why?
Because in vacuum the speed of quantum of light is maximum
and time is stopped, become infinite, unlimited. It means that the
reference frame of vacuum is also infinite, unlimited.
And infinity we cannot measure.
But this doesn’t mean that infinite vacuum doesn’t exist.
We have theories ( thermodynamics and quantum physics) which
explain us the parameters of infinite vacuum.
If we measure the speed of quantum of light in vacuum from
different inertial frames the result will be the *same* - constant.
Because all different inertial frames ( stars and planets of billions
and billions galaxies ) exist in infinite motionless, stationary,
fixed (rest) reference frame of Vacuum.