We explore the effect of vibration on interfacial friction by applying normal and parallel base vibration to a block that rests on an inclined plane. Results show that the block can displace at significant lower angles than the limiting static angle, and that the acceleration level required to cause sliding increases with frequency. Limiting equilibrium analysis is insufficient to explain the observed behavior. Instead, we compute the displacement per cycle and show that (1) the apparent quasi-continuous motion of the block under vibration is the accumulation of successive slip-rest events, and (2) the displacement in every cycle must exceed a threshold displacement in order to cause sliding. The threshold displacement is 0.1 μm for polished granite surfaces; a relation between the threshold displacement and the length scale of surface features is anticipated.