Forgive my ignorance, but I thought a piezo impact sensor was the same thing as a vibration sensor?
While the basic components use the same physical effect they are not the same. An impact sensor is a pair of metal discs with the active element between them. Where as a vibration sensor is a plastic sheet in a small block. It waggles about and produces an AC signal but the sort of vibration it needs is like a washing machine on a spin cycle.