Profiles of the nonlinear optical properties of a two-level system interacting with a thermal reservoir in Four-wave mixing have been studied. The model is a generalization of the conventional two-level approach, it includes a simplified description of the molecular structure and the explicit consideration of permanent dipole moment. The Born-Oppenheimer electronic energy curves consist of two intercrossing harmonic oscillators potentials, displaced both in their equilibrium minimum position and energy. The critical quantities for this analysis are the transition and permanent dipole moments. Our results show how both nonlinear absorption and dispersion responses, are affected by the vibronic coupling.