  Current models of pulsar gamma-ray emission use the magnetic field of a
rotating dipole in vacuum as a first approximation to the shape of
plasma-filled pulsar magnetosphere. In this paper we revisit the question of
gamma-ray light-curve formation in pulsars in order to ascertain the robustness
of the "two-pole caustic" (TPC) and "outer gap" (OG) models based on the vacuum
magnetic field. We point out an inconsistency in the literature on the use of
the relativistic aberration formula, where in several works the vacuum field
was treated as known in the instantaneously corotating frame, rather than in
the laboratory frame. With the corrected formula, we find that the peaks in the
light curves predicted from the TPC model using the vacuum field are less
sharp. The sharpness of the peaks in the OG model is less affected by this
change, but the range of magnetic inclination angles and viewing geometries
resulting in double-peaked light curves is reduced. In a realistic
magnetosphere, the modification of field structure near the light cylinder due
to plasma effects may change the shape of the polar cap and the location of the
emission zones. We study the sensitivity of the light curves to different
shapes of the polar cap for static and retarded vacuum dipole fields. In
particular, we consider polar caps traced by the last open field lines and
compare them to circular polar caps. We find that the TPC model is very
sensitive to the shape of the polar cap, and a circular polar cap can lead to
four peaks of emission. The OG model is less affected by polar cap shapes, but
is subject to big uncertainties of applying the vacuum field near the light
cylinder. We conclude that deviations from vacuum field can lead to large
uncertainties in the pulse shapes, and a more realistic force-free field should
be applied to the study of pulsar high energy emission.
