Can vacuum decay in our universe?

We take a phenomenological approach to the study of 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 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 problem with the effective equation-of-state formalism, we construct the effective dark energy density. Finally, we discuss the evolution of linear perturbation.