Listen to an mp3 clip of B.B. King playing "Worry Worry", and pay attention to the guitarist's vibrato. Notice that although B.B.'s vibrato is very pronounced, he doesn't use it on every note. King reserves vibrato for notes that are held for longer periods of time, or notes he wants to accentuate. Using notes from the blues scale try to mimic King's approach to vibrato.
But, don't take my word for it. Learn about B.B. King's vibrato (and more) from the man himself, in this B.B. King YouTube video guitar lesson.
