  It is a common belief that for magnetic fields typical for accreting neutron
stars in High-Mass X-ray Binaries vacuum polarization only affects the
propagation of polarized emission in the neutron star magnetosphere. We show
that vacuum resonances can significantly alter the emission from the poles of
accreting neutron stars. The effect is similar to vacuum polarization in the
atmospheres of isolated neutron stars and can result in suppression of the
continuum and the cyclotron lines. It is enhanced by magnetic Comptonization in
the hot plasma and proximity to the electron cyclotron resonance. We present
several models to illustrate the vacuum polarization effect for various
optically thick media and discuss how the choice of polarization modes affects
the properties of the emergent radiation by simulating polarized energy- and
angle-dependent radiative transfer. Polarization effects, including vacuum
polarization, crucially alter the emission properties. Together with strongly
angle- and energy- dependent magnetic Comptonization, they result in a complex
spectral shape, which can be described by dips and humps on top of a
power-law-like continuum with high-energy cutoff. These effects provide a
possible explanation for the common necessity of additional broad Gaussian
components and two-component Comptonization models that are used to describe
spectra of accreting X-ray pulsars. We also demonstrate the character of
depolarization introduced by the radiation field's propagation inside the
inhomogeneous emission region.
