The dispersion of the first molecular hyperpolarizability beta of strongly charge-transfer chromophores is investigated. The investigation extends the well-known Oudar-Chemla nonresonant two-level model into the resonant regime. An equation for beta that includes the effect of dephasing and vibrational structure of electronic states is derived. It is shown that if only the dephasing mechanism is included in the two-level model, the intrinsic hyperpolarizability beta(0) calculated using the Oudar-Chemla equation from the hyper-Rayleigh scattering (HRS) experimental data will decrease with decreasing the excitation wavelength. The trend is reversed when the vibrational structure is also incorporated. It is concluded that in order to obtain a consistent beta(0) in the HRS experiment using an excitation wavelength in the resonance regime, it is inadequate to include only the dephasing mechanism. The vibronic structure also plays an important role and needs to be considered in the data analysis in order to obtain a beta(0) to agree with the off-resonance experimental result.