For me, he most important aspect of vibrato is not that it is fast, but that it is relaxed. If you are having pain it could well be because you are clenching between the thumb and forefinger. A good vibrato is one which can transition smoothly between fast and slow, and between wide and narrow. You can practice this by starting with a wide, slow vibrato (taking care that the left thumb is loose, and the forefinger is not tightly held against the neck) then very gradually allowing the vibrato to speed up. This method works for either a wrist, or an arm vibrato. (or a combination)
Brandon