These are my personal thoughts on my vibrato and the players I've been inspired by.
I like it when it's rather wide. More or less one fret in width. It goes from the note and down, most of the time. If I want more tension I may also go a little above the note. It's not so fast, but not too slow. If it's too slow it'll be way to deliberate. The type of vibratoe I'm talking about is very much a throwing thing (as Bob Brozman puts it). You use the weight of the
slide and keep the hand and fingers as relaxed as possible.
Even though you need to first establish the note I do like it when the vibrato starts more or less right away as you hit the note, and that it's full right away too. This makes for a fatter sound. Tampa Red taught me this as I listened to recordings of his vibrato. It's like it's always there, he just turns it off fromo time to time.
If you can't hear the others you're too loud, if you can't hear yourself you've gone deaf