The low-frequency vibrations of the Boson peak are observed with inelastic neutron and Raman scattering in a homologous series of polycarbonate copolymers. In accordance with the notion that the Boson peak is predominantly an acoustic phenomenon, the Boson peak energy is found to correlate with the velocity of sound. However, a striking inverse correlation is observed between the Boson peak energy and the average size of the unoccupied volume elements (approximately 5-6 Angstrom in diameter) evidenced by positron annihilation lifetime spectroscopy (PALS). The apparent correlation between the Boson peak energy and PALS data is discussed in terms of a relationship between the elastic modulus and the electron density heterogeneities.