The vibrations are the start, but it's not really what happens the moment you leave. I'd like to think of the vibrations as "preparing" yourself for entry into the astral dimension. In Robert Monroe's book, he suggest at first, with your mind, feel around the room. So, for example, you are laying on a couch and nearby is a desk with a water bottle. Use your influence to feel the bottle. If you need to, lift your arm towards the bottle and begin to feel it. Hold it in your hand and massage it, feel the temperature difference, etc. Do this a couple of times. That helped me prepare to leave as it gives you a good practice on the "feeling" aspect of things. Dunno if that makes sense.