Surface-enhanced Raman scattering (SERS) allows for detection and identification of molecular vibrational fingerprints in minute sample quantities. The SERS process can also be exploited for optical manipulation of molecular vibrations. We present a quantum description of surface-enhanced resonant Raman scattering, in analogy to hybrid cavity optomechanics, and compare the resonant situation with the off-resonant SERS. Our model predicts the existence of a regime of coherent interaction between electronic and vibrational degrees of freedom of a molecule, mediated by a plasmonic nanocavity. This coherent mechanism can be achieved by parametrically tuning the frequency and intensity of the incident pumping laser and is related to the optomechanical pumping of molecular vibrations. We find that vibrational pumping is able to selectively activate a particular vibrational mode, thus providing a mechanism to control its population and drive plasmon-assisted chemistry.