Vibration-based energy harvesters have been extensively studied and investigated to harvest the energy produced by environmental mechanical vibration sources as mean to produce low electrical energy, thereby supplying low-power sensors and actuators. Different devices have been proposed as energy harvesters, cantilevers-based geometries have been pursued frequently in the literature. Here, we propose the geometry of an elastomeric circular membrane coupled with an electret (soft electrostatic generator) with a central proof mass. By soliciting the designed device around its resonance frequency of 14Hz with an acceleration of 0.4g for a mass of 9.5g, the system produced an average electric power of 24μW for an optimal resistance of 150MΩ. An analytical study developed closely with a finite element simulation with Comsol® allowed to validate the obtained experimental results, suggesting that this approach can be used as a tuning method to develop other geometrical shapes and conceive large-scale devices for vibration energy harvesting applications.