i don't really know of any specific excercises for working on vibrato, i'd say a lot of songs by steve vai or jason becker would be good for practicing? i just seemed to get better over time because of learning random songs.
I like to just have fun with my vibrato. The only thing I really wanna "learn" how to do is the crazy Whammy bend at the beginning of Erotic Nightmares by Steve Vai. It is the coolest use of whammy bar ever
A good way to "practice" vibrato is to exaggerate it.. just practice on one fretted note at a time , really lay the vibrato on thick ,.. then just practice changing positions at a faster tempo while still placing emphasis on the vibrato.
I find bar vibrato good for extreme changes in the way a note is played, but subtle great vibrato I still believe comes from the fingers.
I like to just have fun with my vibrato. The only thing I really wanna "learn" how to do is the crazy Whammy bend at the beginning of Erotic Nightmares by Steve Vai. It is the coolest use of whammy bar ever