Vibrational spectra of Ga2S3-CdS solid solutions are studied by far-infrared and Raman scattering spectroscopy. All the spectra can be interpreted using a model considering the decoupled weakly deformed MX4 tetrahedral units. The dominant Raman scattering band attributed to be breathing A1-mode originates from the vibration of the four neighboring atoms of the vacancy. The broadening of the Raman bands increases with the addition of cadmium atoms in Ga2S3-CdS solid solutions. This effect is attributed to disordering of the gallium positions in the cation sublattice. A band attributed to the presence of weakly deformed CdS4 tetrahedral units corresponds to the fundamental A1 mode of CdS. Raman scattering and infrared absorption spectra of CdGa2S4 crystals have been reconsidered for the alpha- and beta-form. This work clearifies the confusion contained in the literature on the lattice vibrational spectra of the two forms of CdGa2S4.