We present a rigorous theory for oblique incident Hermite-Gaussian beams, diffracted by two optical nano-slits of width l and separation d, in a thick metallic screen for the case of polarization TM (P). The far field spectra as a function of several opto-geometrical parameters, wavelength λ , slit width l, separation d , incidence angle θi and Hermite order m is analyzed. In the vectorial diffraction region given when λ /l >0.2, where l is the incident wavelength and as a function of the separation between slits d; we have numerically analyzed: the far field spectra, the energy diffracted along the incident beam direction Ei , and the validity of an approximate diffraction (scalar) property, namely Ei= Ntao/lambda .