Thanks Jim,

The only problem with that kind of open string vibrato is that the pitch will be slightly high. Not by much since you are only doing a little but still. This is fine if you vibrate above the pitch on all the other notes as well but not if you have the highest point of the vibrato as the defined pitch.

Another way to vibrate open strings is to vibrate the fourth finger on the string below while playing the open string. It actually gives you a vibrato on that open string. It is commonly done.