This paper presents a simple analytical method to optimize the efficiency of two types of electrostatic Vibration Energy Harvesters (VEH): the out-of-plane (OPGC) and in-plane (IPGC) gap-closing converters. For the first time the electrical and mechanical behaviours of the transducer are addressed simultaneously, while a voltage limitation on the transducer’s terminals is set to prevent any damage in the conditioning electronic. The presented work allows to the designer to determine the best strategy depending on whereas the system is passive or able to be self-adapted to the external vibrations parameters. The calculations are validated by VHDL-AMS/ELDO simulations.