The vibrato in this case is usually to bend a note up, say, one fret, then let it relax and re-tension it a few times. You don't always have to let it back to the ordinary fretted note, it could be relaxed half a fret down, but then it only needs to be bent back to the target note. Again, with a two fret bend that is given vibrato, it may need to go to a straight string again, but it may only need to be relaxed by half a semitone, or one semitone.Try to use a couple of fingers, it's hard work controlling a big bend with only one. It depends what style, genre or who you're trying to emulate. You need to do it slowly to check pitch, particularly the target note. Don't overshoot. Ears are good for this. Yours or your listeners.A tuner will also help, when practising slow vib. As often, no substitute for doing it lots, a.k.a. practice.