Films of electret material are currently used to cover the surface of electrodes of some vibration capacitive harvesters based on deformable beams, clamped at both ends. Nevertheless, performance of this device is often predicted through some simplified electromechanical model, which neglects the effect of geometric nonlinearity due to a mechanical coupling between the axial and flexural behaviors of the clamped beam. Stiffening of beam and nonlinear behaviour in dynamics is herein investigated, by resorting to a distributed model of the electromechanical copuling of the vibration harvester, based on the Finite Element Method. Influence upon the performance of the energy conversion is then analyzed and an optimization of the configuration is proposed.