We examine the vibrational density of states of atomistic models of polypropylene glasses containing a single impurity molecule of s-tetrazine. We discuss existing methods and develop new ones to achieve significant data reduction and navigate through the complex spectrum of the normal modes of the glass. By calculating the participation ratio, the distribution of the kinetic energy of each mode on the atomic coordinates, and a mode-proximity index to the solute it is possible to identify impurity-related, polymer-related, and mixed modes and assess their relative contributions to the vibrational density of states. Activation of specific modes using molecular dynamics allows the observation of anharmonicities in the doped glass, even at very low temperatures.