  The cosmological vacuum decay scenario recently proposed by Wang and Meng
\cite{wm} is rediscussed. From thermodynamic arguments it is found that the
$\epsilon$ parameter quantifying the vacuum decay rate must be positive in the
presence of particle creation. If there is no particle creation, the proper
mass of Cold Dark Matter (CDM) particles is necessarily a time dependent
quantity, scaling as $m(t) = m_o a(t)^{\epsilon}$. By considering the presence
of baryons in the cosmological scenario, it is also shown that their dynamic
effect is to alter the transition redshift $z_*$ (the redshift at which the
Universe switches from decelerating to accelerating expansion), predicting
values of $z_*$ compatible with current estimates based on type Ia supernova.
In order to constrain the $\Omega_m - \epsilon$ plane, a joint statistical
analysis involving the current supernovae observations, gas mass fraction
measurements in galaxy clusters and CMB data is performed. At 95% c.l. it is
found that the vacuum decay rate parameter lies on the interval $\epsilon =
0.11 \pm 0.12$). The possibility of a vacuum decay into photons is also
analyzed. In this case, the energy density of the radiation fluid scales as
$\rho_r = \rho_{ro}a^{-4 + \epsilon}$, and its temperature evolution law obeys
$T(t) = T_oa(t)^{\epsilon/4 - 1}$.
