Vibration energy harvesting is a promising method to feed ultra-low power devices as wireless sensors, mems, etc. The accuracy of the design of the harvesting device, and then the harvested power, strongly depends on the modeling quality of the magnetostrictive relationships and on the device modeling in general. A further important issue is the optimization of the converted power with respect to the frequency of the vibration source, which is critical for low frequencies. The use of a capacitor in the electric circuit allows more harvested power at low frequencies and open the possibility of power optimizing stages. This analysis could be reliably carried out only in connection to a sufficiently realistic modeling of the employed material. Aim of the paper is therefore to study, by the definition of a simple nonlinear model, the effects of the capacitive load and of the mechanical source on the power conversion performances of the device. Several numerical examples are presented and discussed.