  The mass of the neutron star in Vela X-1 has been found to be more massive
than the canonical 1.5 Mo. This result relies on the assumption that the
amplitude of the optical component's measured radial velocity curve is not
seriously affected by the interactions in the system. In this paper we explore
the effect on the radial velocity curve caused by surface motions excited by
tidal interactions. We use a calculation from first principles that involves
solving the equations of motion of a Lagrangian grid of surface elements. The
velocities on the visible surface of the star are projected along the
line-of-sight to the observer to obtain the absorption-line profile in the
observer's reference frame. The centroid of the line-profiles for different
orbital phases is then measured and a simulated RV curve constructed. Models
are run for the "standard" (vsini=116 km/s) and "slow" (56 km/s) supergiant
rotation velocities. We find that the surface velocity field is complex and
includes fast, small-spatial scale structures. It leads to strong variability
in the photospheric line profiles which, in turn, causes significant deviations
from a Keplerian RV curve. The peak-to-peak amplitudes of model RV curves are
in all cases larger than the amplitude of the orbital motion. Keplerian fits to
RV curves obtained with the "standard" rotation velocity imply a neutron star
>1.7 Mo. However, a similar analysis of the "slow" rotational velocity models
allows for m_ns ~ 1.5 Mo. Thus, the stellar rotation plays an important role in
determining the characteristics of the perturbed RV curve. Given the
observational uncertainty in GP Vel's projected rotation velocity and the
strong perturbations seen in the published and the model RV curves, we are
unable to rule out a small (~1.5 Mo) mass for the neutron star companion.
