Based on double harmonic oscillator (DHO) approximation, the static vibrational first hyperpolarizabilities,and the IR and Raman spectra of 4-N-methylstilbazonium salt derivatives are calculated using coupled perturbed Hartree-Fock (CPHF) approach. Two vibrational normal modes which dominate the contribution to the vibrational first hyperpolarizabilities are obtained. It is found that the static vibrational first hyperpolarizabilities of title molecules are large and show a good linear relationship with their electronic counterparts. For the first time we give an approach of estimating the vibrational first hyperpolarizabilities using a few normal modes identified from IR and Raman spectral peaks, which the name as the few mode approach. The results indicate that this approach is practical in the estimation of the vibrational first hyperpolarizability.