We report on the wavelength-dependent nonlinear-gain properties of a vertical-cavity semiconductor optical amplifier. A step-like output versus input transfer curve and bistability are found on the long wavelength side of the resonant peak. The characteristics are caused by a dispersive nonlinearity arising from gain saturation in the device. The nonlinear transfer of the amplifier could be used for logic regeneration and all-optical logic operations, while the optical injection power required to switch the device is an order of magnitude lower than previously reported with edge-emitting devices.