We compute the fluorescence spectrum of a molecular aggregate with electronic and vibrational degrees of freedoms, described by Holstein model, in an optical cavity. The redistribution of oscillator strength among the vibronic polaritons due to strong interaction with the cavity mode and its effect on superradiance is investigated. Simulations demonstrate the scaling of spectra with the coupling strength to cavity and with aggregate size. (C) 2017 Elsevier B.V. All rights reserved.