This paper presents an analysis, behavioral modeling and functional design of a capacitive vibration energy harvester, composed from a mechanical resonator, capacitive transducer and a conditioning circuit based on the BUCK DC-DC converter architecture. The goal of the study is to identify the optimal scenario of the BUCK switch commutation and to propose a functional model of the switch device. The study included an analysis, design and modeling phases. We found that the optimal commutation timing is defined by the energetical state of the circuit (voltages and currents) rather by a "hard" time specification. The developed VHDL-AMS functional model of the switch was validated by a ADvanceMS multi-physics simulation of the whole harvester. The resonator and capacitive transducer were modeled in VHDL-AMS, the electric elements of the conditioning circuit by their ELDO models.