We present a classical description of femtosecond time-resolved experiments on collision-induced vibrational relaxation processes. For I-2 embedded in Ar and He gases, a hot distribution of molecular vibrational states is prepared by a pump laser pulse. Collisions of I-2 with gas atoms lead to dephasing and energy relaxation in the vibrational degree-of-freedom. In employing energy-phase coordinates, we are able to separate these two effects. Decay rates as obtained from the Fourier analysis of pump-probe signals are compared to those derived from the coordinate autocorrelation-function. It is shown that the vibrational energy relaxation influences the decay of the oscillatory structure in transient signals, leading to a nontrivial density dependence of the dephasing rate as observed in the experiments of Liu et al.