Keep your fingers solidly behind the ring finger on the fretboard (left hand) and fret one note. Gradually bend the string and stop it at whatever pitch then bend it some more and stop. Try to play definite notes in this manner. STOP when you hit the note you want.Take it up a tone and STOP. This is like Kegel's exercise
and will help you be stable on every note on a bend.To achieve a smooth vibrato exaggerate every mechanical move and master it.Always remember that the start and end notes of a bend have to relevant to the melodic idea you are on. The oscillation of a string to sustain it has to be smooth and musical. This is very critical and separates a musician from a guitar player.