A simple method for simulating two-dimensional (2-D) distributions of the diffraction lights of a volumeholographic- type VanderLugt correlator is proposed and demonstrated. The simulation results are similar to those of the corresponding experiments, and only a few dozen points are sampled from both the input and the reading patterns. We show that the shifting tolerance to the reading pattern is a result of Bragg degeneracy and is not isotropic. The Bragg degeneracy generates different degrees of cross talk between the horizontal and the vertical directions. With the method we further simulate the 2-D shifting tolerance of the volume-holographic correlator. The simulation results offer a clear picture of the diffraction in a transmission volume hologram used as an optical spatial correlator.