A feeling of vibration is caused by your brain receiving both nonphysical and physical signals at the same time. Think of it like binaural beats. If you play a tone with a frequency of 500 hertz in one ear it's a solid sound. If you play a sound of 520 hertz in the other ear then you also hear a solid sound. If you play 500 in the left and 520 in the right however you will hear a "beat" or dynamically changing sound that goes up and down very quickly. This happens because your brain is receiving two signals and trying to convert them into one.
Let's move back to what you experienced... You had physical focus and as your brain shut down to sleep you began moving into nonphysical focus. This caused a slight collision of your sense of touch signals which resulted in weak vibrations. When you relaxed into them your brain thought you were falling asleep and started shuting down and disabling your body to prevent you moving physically while dreaming. This resulted in increased nonphysical senses which overlapped your physical senses ever further thus making the vibrations stronger.
Finally you focused on the strong vibrations, possibly began analyzing the feeling of them and got a little excited. Your brain noticed that you were conscious and was no longer fooled by your attempt to trick it into thinking you were asleep so it re-enabled your body causing your physical senses to return and your nonphysical senses to fade away. This resulted with the vibrations disappearing entirely since your brain was no longer recieving nonphysical and physical senses at the same time.
Next time don't notice the vibrations or your body. Instead you should concentrate on a nonphysical environment or another object in your room so that you can be sucked into it. Maybe try levitating or rolling out of your body as long as you are certain your out of physical focus. Once you are entirely in nonphysical focus you will no longer feel the vibrations since you are completely disconnected from your physical senses. So if your feeling vibrations wait for them to pass but don't focus on them or your body.
Hope I helped and good luck