The vibrational relaxation of an I-2 molecule in a xenon fluid has been simulated by three methods. The conventional perturbation theory approach based on an equilibrium molecular dynamics (MD) simulation is compared with purely classical nonequilibrium MD and mixed quantum-classical surface-hopping MD simulations. Relaxation times and state-to-state vibrational transition rate constants obtained with these three approaches are compared and information that can be extracted concerning the mechanism of vibrational relaxation is examined. Both harmonic and anharmonic solutes are considered as are some common approximations invoked in obtaining vibrational relaxation times.