Set the tempo of your metronome to 60 beats per minute, then start playing the note you wish to learn to vibrate on either your keyboard or guitar. Pick a key around the middle of your particular vocal range, as you will move five or more notes up and down the vocal scale during this exercise. This will be the base note in which your voice will play to while singing during this exercise.
Sing a note one half-step higher than the one you are playing on the guitar or keyboard, then sing the actual note that the instrument is playing. For example, play a sustained C note with your instrument, a C sharp note with your voice, then sing a C note. Sustain the notes being played with your voice as you move through the scale. Check to see that you are hitting the correct note with your voice by briefly hitting the next highest note on the instrument, then going back to the note being played instrument for the exercise. Repeat this step three times.
Move up the vocal scale to strengthen your voice for vibrato technique. Play the next highest note on the keyboard or guitar while singing a note one half-step higher than the one the instrument is playing, then lower the pitch of your voice to the note being sustained by the instrument. For example, move from C to C sharp on the instrument, then hit a D note with your voice. Increase the metronome's speed by 10 BPM as well. Repeat this step three times, then continue to move upward on the scale until you have done this for at least five notes from where you began.
Practice this exercise in the other direction. Repeat step three while moving down one half-step each time, until you reach five notes lower from where you first began. Continue to increase the speed of the metronome 10 BPM as well time you move down the scale. Within two or three weeks of practicing this routine daily, you will notice a significant difference in your ability to control your voice while vibrating a note.