Fretless vibrato is traditionally an elbow motion. On bass and guitar, you move the string up and down on the fret with your palm and fingers. On violin, cello and other bowed instruments, you vibrate the pitch with your wrist ever so slightly. With a fretless bass I, and many others, perfer to use the entire underarm. Hold your arm with a straight wrist, and move the arm right and left from the elbow and up.