Quantum transport through single molecules is essentially affected by molecular vibrations. We investigate the behavior of the molecular transistor with intermediate electron-vibron coupling and arbitrary coupling to the leads. We have developed a theory which allows to explore this regime via the nonequilibrium Green function formalism parallel to the widely used master equation technique. The problem is motivated by recent scanning tunneling spectroscopy experiments. We show that the nonequilibrium resonant spectroscopy is able to determine the energies of molecular orbitals and the spectrum of molecular vibrations. Our results are relevant to STS experiments, and demonstrate the importance of the systematic and self-consistent investigation of the effects of the vibronic dynamics onto the transport through single molecules.