  We take a phenomenological approach to study the cosmological evolution of
decaying vacuum cosmology ($\Lambda(t)$CDM) based on a simple assumption about
the form of the modified matter expansion rate. In this framework, almost all
the current vacuum decaying models can be unified in a simple manner. We argue
that the idea of letting vacuum decay to resolve the fine-tuning problem is
inconsistent with cosmological observations. We also discuss some issues in
confronting $\Lambda(t)$CDM with observation. Using the effective equation of
state formalism, we indicate that $\Lambda(t)$CDM is a possible candidate for
phantom cosmology. Moreover, confronted with a possible trouble of effective
equation of state formalism, we construct the effect dark energy density.
Finally, we discuss the evolution of linear perturbation.
