In this work we have studied the vibrational (relaxation) contributions beta(r) and gamma(r) to molecular nonlinear optical (nlo) properties of polyenovanillins, a class of push-pull molecules with increasing number n of conjugated CC bonds (n = 1, 3, 4, 5). Both experiments and ab initio calculations have been carried out. The model adopted expresses the nuclear relaxation contributions in terms of vibrational spectroscopic observables, namely frequencies and infrared and Raman intensities. A relevant observation is that the electronic hyperpolarizability beta(e), measured from EFISH experiments or calculated by ab initio methods, turns out to be very similar to the experimental or ab initio vibrational beta(r). The same behavior has already been found (both for beta and gamma) in other polyconjugated low band-gap materials with large electron-phonon coupling. Moreover, the spectroscopic approach has the advantage of allowing useful insight into the role played by structural parameters in the nlo processes.