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We study the electromagnetic properties of plasmas allowing for the polarization effect of the vacuum induced by the presence of a strong external magnetic field. Adopting the cold-plasma polarization tensor to account for the electron component, we find that the polarization properties and transport of x-ray radiation can be severely altered by the magnetic vacuum effects. The effect becomes important at relatively high frequencies, o/w, Z (15n/a)'" B,/B where o, = (4nn, e2/m)"2 is the plasma frequency of the electron component, a is the fine-structure constant, and B, = m2c3/e+i=4.4 X loi3 G is the electrodynamic critical field. For typical pulsar magnetic fields, B -0.1 B,, and x-ray frequencies, this condition is satisfied even for relatively high plasma densities n, -lo2' cm 3 . Whenever the above inequality is fulfilled the vacuum effect dominates the polarization properties of the normal modes of the medium, giving rise to a significant change in the medium opacity. The Thomson, bremsstrahlung, and cyclotron opacities are thus significantly altered from their usual cold-plasma values. The largest departures are found in the vicinity of the electron gyrofrequency, where the vacuum induces a strongly anisotropic resonant behavior on the ordinary mode (which is nonresonant in the usual cold-plasma circumstances). This property may be significant in analyzing the polarization and spectral character of the cyclotron feature in Hercules X-1 and other accreting x-ray pulsars.