We present a work on the design and the fabrication by Molecular Beam Epitaxy of a monolithic vertically integrated VCSEL and Electro-Absorption modulator (EAM). This complex optical multilayer stack combines two optical cavities, between which the coupling strength requires to be precisely controlled. Indeed, the final performances of the EAM-VCSEL device, in terms of frequency modulation response, is closely correlated to this coupling, in addition to the wavelength detuning between each cavity resonances and respectively the VCSEL gain and EAM multi-quantum wells absorption spectra. Our work will give clear insights on the required accuracy for the epitaxy of such EAM-VCSEL structures and on the in-situ measurement methods to implement to reach the best performances.