Watch the Adding the 9th and 13th online guitar lesson by Fareed Haque from Jazz Rock Workshop
There are many types of vibrato. BB King has his "shake," Segovia has his vibrato, Ted Nugent has that thing he does where he gets his 3rd finger under the string, like at the end of "Stranglehold" (try that at home at your own risk because I'm pretty sure you can only get that kind of vibrato with a bow and arrow, or a 12-inch disemboweling knife).
Try experimenting with side-to-side vibrato for a more classical, elegant sound. Up and down vibrato for a more "bluesy" shake, usually pulling the string, then pushing it.
You should experiment using the whole hand to shake the string, then try it with just the finger. Work on vibrato with 1st, 2nd, 3rd and 4th fingers.