In this paper, Virtual Cathode Oscillator as a microwave generator is studied concentrating on the nonlinear behavior of the microwave-plasma interaction that introduces chaos to the output voltage and power of the generator. It is shown that periodic oscillations of the electron beam strictly depend on the two most critical characteristics of the VCO, the input maximum voltage and the cathode radius. The chaos caused by the critical values of these two parameters, could restrict the maximum possible values of the input parameters, reducing the overall maximum output values. The effects of these two parameters are studied trough PIC-code simulations. The simulation results were used to extract the bifurcation diagrams and analyze the chaos patterns accurately. Chaos, as a serious cause for overall efficiency and output power reduction, is decreased applying a new method which is proposed considering the study results.