In apertureless scanning near-field optical microscopy (ASNOM), the probe vibration is often used to increase the detected signal. The useful signal is detected at the probe-vibration frequency by a lock-in amplifier. By comparing two-dimensional numerical results with an experimental scan, we show numerically that, to explain or predict the detected signal, a realistic model of ASNOM should take into account the scan of the probe as well as the probe vibration and the material properties.