Those "vibrations" are called Hypnic (aka Hypnagogic) Jerks. Which are also normal occurances in the sleep cycle, they ussually however occur in the later stages of falling asleep and early stages of waking up, when we aren't conscious enough to remember them.

They are often described as a falling sensation or quick jolts. The falling sensation is ussually coupled(at least for me anyway) with images of actually falling as you described.