Shaking your head??? That's a new one for me, dewd. But if it works for you, kool. The vibrato effect is done by controlling the contractions of the diaphragm muscle. Which your supposed to tighten when you sing anyways. It takes a lot of practice to actually gain control, but what you do is release and contract the muscle repeatedly to get the effect. It's pretty hard to do at first. You can do it artificially by placing the palm of your hand just below your sternum and sing a pitch. Hold it and vibrate your hand on you chest. That's it. Doing it without using your hand is the hard part. Breathing exercises help develop the technique.

You can get higher and lower in pitch just with practice. Once you can sing the highest pitch easily, and your vocals get comfortable with that pitch. You should be able to move up. It's just like going up steps. Takes some amount of time though.