This paper studies the vibration energy harvesting (VEH) capabilities for one electrostatic mechanism: the out-of-plane gap-closing (OPGC) converter. The goal is to maximize the output energy harvested from a MEMS device while preventing the integrity of the conditioning circuit. The originality of this work lies in the optimization taking into account both electrical and mechanical aspects and studying at the same time the consequence of the voltage limitations on the transducer. After a reminder of working principle of the VEH at constant charge, we'll propose an optimal design of the structure, analysing the consequences of an external acceleration's fluctuation and a way to remedy this with two different approaches. Also, we'll study the influence of the additional parallel (parasitic) capacitance.