The effect of high-frequency vibrations on the nonlinear regimes of thermal convection in a two-layer system composed of a horizontal pure fluid layer and fluid-saturated porous layer heated from below is studied in the framework of the average approach. For large porous layer thicknesses it has been found, that at low vibration intensities the evolution of convective regimes with the growth of the Rayleigh number proceeds as follows: stationary regime-oscillatory regime-stationary regime. At high vibration intensities the stationary convective regimes take place at any values of the supercriticality used in the calculations. At close values of the fluid layer and porous layer thicknesses the interaction between the short-wave and long-wave instability modes is investigated. It has been found that at high vibration intensities the ambiguity of the stationary solutions is observed in a certain range of supercriticalities.