When a "real" particle randomly falls towards a very low energetic threshold, it is "trapped" into the quantum vacuum as a quantum oscillation. This leads to a progressive vacuum volume increase (and to the expansion of the Universe), while the vacuum energy density is kept constant. Therefore, free energy=real particles, entropy= the energy trapped in the vacuum. This also means that at the Big Bang there was no vacuum into our 4D spacetime manifold.
For further details, see:
https://www.researchgate.net/publication/315675113_OUR_UNIVERSE_QUANTUM_VACUUM_AND_ENTROPY_WHO_COMES_FIRST
Preprint OUR UNIVERSE, QUANTUM VACUUM AND ENTROPY: WHO COMES FIRST?