We suggest a simple quasiclassical method for the estimation of inelastic scattering length for the exoergic VT energy transfer at ultra-low energies. The non-quasiclassical nature of inelastic collision is fully accounted for by incorporation of the elastic scattering length into the expression for the inelastic scattering length. Other factors are the quasiclassical Landau transition probability and the classical vibrational frequency of the van der Waals complex. The method gives a simple interpretation of the large difference in inelastic scattering lengths for the vibrational deactivation of hydrogen molecule in collisions with He-4 and He-3 found in accurate numerical calculations.