There are two very different sets of vibrations I have experienced.
One, is a feeling of electricity shooting down the legs. The is VERY unmistakeable.
The other I experience is described by Yuschak in his book on LDS. This is also a very profound experience and one I have often. I start to feel my head and neck vibrate. It starts slow and can build to a very intense feeling. I am sure that it's a matter of perception and that my head and neck are stationary. The sensation feels like it goes on forever, but in reality, it might be 10-15 seconds. Usually when the sensation ends, I am dreaming. So it's a transitionary event (wakeful state to dreamstate).