Watch the Vibrato online guitar lesson by Steve Vai from Alien Guitar Secrets: Passion & Warfare
Make the determination today to become a master of vibrato - to break it down and take the time - because the payoff is so great when you have complete control over it. There are various vibrato techniques, none that I would recommend over another, just whatever is comfortable for you. The classic rock and roll vibrato is where you take the note, bend it a little sharp, with various widths you can play. There's classical style vibrato, which is more of a forward/backwards, which is nice because it makes a note go flat and then sharp, again and again. With the rock vibrato style, most people don't actually hit the note, they just stay sharp. The classical vibrato will give you a more rounded style of vibrato, but it is somewhat limited (and doesn't look cool). I developed a type of vibrato, although I'm not sure how it came about, which is more circular and I push and pull to go both sharp and flat. I find it useful for various speeds and aggressions. I would sit endlessly for hours experimenting with every type of vibrato. I might recommend first looking at your vibrato and critiquing it. Alongside this, it's important to record what you play, and listen back to it very carefully to see what you like (and be careful to not to tell yourself that you suck). Once you've identified it, then move in on it. Identify it and get excited about it.
If you really want to be a master of vibrato, you need to know about a few different things: There's very delicate vibrato, achieved by practicing how slow you can hit a note. Also try out going from slow to fast. And then there's different widths - with which you can use different speeds. You can play vibrato fast and shallow, or slow and wide, and vice versa. There's also vibrating notes while you bend them, with the same rules applying. Also practice vibrating with every finger. Play for an hour with one finger on one string, focusing on every depth and speed of vibrato you can possible think of, and you'll become a master of vibrato. Then, you can start getting into things like double stops; vibrating two notes at the same time. Even three notes - going fast and slow. It's hard to get every note to sing when doing this. When you have vibrato at your command, it's very rewarding.