A representation of the diatomic vibrational distribution function in energy and phase variables yields a transparent physical picture of collision-induced vibrational relaxation. In this way, the energy- and phase-relaxation processes can be separated. A simple parametrization of the phase distribution allows for the extraction of the dephasing time and the reconstruction of autocorrelation functions and femtosecond pump-probe signals. It is shown that the dephasing time can be derived from the Fourier spectrum of the pump-probe signal in favorable cases.