This paper presents optimization, design and modeling of a conditionning circuit of a vibrational energy harvester with capacitive electromechanical transducer. The conditionning circuit is inspired from the Buck DC-DC converter architecture, and composed from a charge pump and a flyback circuit. We found that the switching should be ordered by the internal state of the circuit, an not by some fixed timing scenario. The paper presents how to find the optimal operation mode of the harvester. To validate the study, the system was modeled using a mixed VHDL-AMS - ELDO model.