We theoretically study the photoluminescence properties of organic-based microcavities with vibronic progressions. We analyze the relaxation from the exciton reservoirs to the polariton anticrossing region; the presence of vibronic levels qualitatively enriches the physics of the system with the appearance of new decay channels. The relaxation dynamics is studied with a master equation and the cavity photoluminescence is obtained from the polariton stationary population. We show that the exciton radiative recombination in which a molecule ends up in a vibrationally excited level of the electronic ground state can lead to qualitatively new features in the cavity luminescence spectrum.