Reduce the amount of air pressure you use to sustain a note. Unlike projecting the voice, which requires a strong breath to support the sound, vocal vibrato requires the muscles to be relaxed. Make sure that you are not tightening your abdominal muscles or forcing air from your stomach, both of which can increase the tension in your neck, making vocal vibrato difficult to achieve.
Stand in front of a mirror and watch yourself as you sing. Observe what the muscles in your neck are doing as you climb an octave, sing louder or increase the amount of air pressure. If your neck muscles bulge or your voice feels strained, sing in a lower register, relax your body and decrease the amount of air pressure until you have achieved vocal vibrato.
Sing low and soft when first trying to produce a vocal vibrato. Most singers cannot produce this effect naturally, so you must practice it in order to sing with a vocal vibrato.