  In the present days of modern cosmology it is assumed that the main
ingredient to cosmic energy presently is vacuum energy with an energy density
$\epsilon_\mathrm{vac}$ that is constant over the cosmic evolution. In this
paper here we show, however, that this assumption of constant vacuum energy
density is unphysical, since it conflicts with the requirements of cosmic
thermodynamics. We start from the total vacuum energy including the negatively
valued gravitational binding energy and show that cosmic thermodynamics then
requires that the cosmic vacuum energy density can only vary with cosmic scale
$R=R(t)$ according to $\epsilon _\mathrm{vac}\sim R^{-\nu }$ with only two
values of $\nu$ being allowed, namely $\nu _\mathrm{1}=2$ and $\nu
_\mathrm{2}=5/2$. We then discuss these two remaining solutions and find, when
requiring a universe with a constant total energy, that the only allowed power
index is $\nu _\mathrm{1}=2$. We discuss the consequences of this scaling of
$\epsilon _\mathrm{vac}$ and show the results for a cosmic scale evolution of a
quasi-empty universe like the one that we are presently faced by.
