Vibrato is one of the most, if not the most, expressive tools that we guitarists have in our arsenal of techniques. Continuous musical phrases without the potent thrill of vibrato sound less dramatic than phrases with the right vibrato used in the right places and situations. Classical singers spend many years and a lot of effort trying to master vibrato with their voice. Many guitarists, however, don't spend much time trying to master this technique at all. Many guitar teachers either don't value vibrato much themselves or just take the technique for granted and do not stress the importance of vibrato enough to their students. It is unfortunate that the value of possessing a great vibrato technique is often underestimated.
I believe that the best and most expressive vibrato is slow to moderate in speed but wide in range. There are times, however, when a somewhat narrower range is needed (usually over slow and soft musical passages). A fast vibrato can sometimes be effective, but use it with caution as it can lead to an out of control sound (and an out of tune sound if you are not careful). I very rarely use a fast or narrow vibrato and my vibrato is never fast and narrow at the same time!
Here are the most common problems that many inexperienced guitarists have with their vibrato technique:
I highly recommend listening to players that have an impeccable vibrato technique. Here is a short list of virtuosos that have helped me to shape my own vibrato: Yngwie Malmsteen, George Bellas, Andy LaRocque (plays in the King Diamond band) and Jason Becker. There are, of course, other players who I think have excellent vibrato too -s here are a few more guitarist's vibratos to listen for: John Petrucci, Marty Friedman, George Lynch and Eddie Van Halen (on the older Van Halen recordings). A great vibrato technique usually takes a long time to master. It wasn't until after playing for many years that I felt that my own vibrato was where I wanted it to be. Finally, I'd like to suggest to you not to overlook or underestimate the importance and effectiveness of this highly expressive technique. Even after you master it, constantly work on it and continue to refine it always.