  We report pronounced modifications of electronic and vibrational properties
induced in FeO by cation vacancies, obtained within density functional theory
incorporating strong local Coulomb interactions at Fe atoms. The insulating gap
of FeO is reduced by about 50% due to unoccupied electronic bands introduced by
trivalent Fe ions stabilized by cation vacancies. The changes in the electronic
structure along with atomic displacements induced by cation vacancies affect
strongly phonon dispersions via modified force constants, including those at
atoms beyond nearest neighbors of defects. We demonstrate that theoretical
phonon dispersions and their densities of states reproduce the results of
inelastic neutron and nuclear resonant x-ray scattering experiments \emph{only}
when Fe vacancies and Coulomb interaction $U$ are both included explicitly in
\emph{ab initio} simulations, which also suggests that the electron-phonon
coupling in FeO is strong.
