Today I learned that wrist vibrato is initiated by the wrist, and the fingers go along for the ride, so to speak. The fingers have to be flexible so they can stay on whatever note but still let the wrist and hand move, but the fingers don't start the the vibrato or keep it going. That would be like the tail wagging the dog.
Why do I say I learned this today, when it's only what every teacher + every article + every book + every video + Simon Fischer and Nate Cole and all the rest of the stringplaying world have been telling me for like 8 years? I've been focusing on vibrato for the past several weeks, going back to basic drills and trying some new ones. After all this time, I finally have enough coordination to actually watch what's happening while I listen. So after all these years I finally put two and two together and learned what I already knew.