  The thermodynamic behavior of a decaying vacuum cosmology describing the
entire cosmological history evolving between two extreme (early and late time)
de Sitter eras is investigated. The thermal evolution from the early de Sitter
to the radiation phase is discussed in detail. The temperature evolution law
and the increasing entropy function are analytically determined. The entropy of
the effectively massless particles is initially zero but evolves continuously
to the present day maximum value within the current Hubble radius, $S_0 \sim
10^{88}$ in natural units. By using the Gibbons-Hawking temperature relation
for the de Sitter spacetime, it is found that the ratio between the primeval
and the late time vacuum energy densities is $\rho_{vI}/\rho_{v0} \sim
10^{123}$, as required by some naive estimates from quantum field theory.
