In conventional side match vector quantizers (SMVQs) for images, the associative side match errors at both the horizontal and vertical boundaries are sorted and then used to construct the state codebooks. In this study, a competitively boundary error sorting (CBES) scheme, in which the separate side match errors at two boundaries are competitively sorted, is proposed to design a better next-state function for SMVQs. The simulation results show that the coding performance and, especially, the visual quality can be improved for the images with strong edges.