Start with all four fingers down, use where the not the pinky is on then add vibrato, go as wide as you can at a medium to fast speed. A whole step or more if the most melodic generally, IMO, but...

Start with all 4 down, the 4 fingers will help the pinky with the virbato, take the pinky off, then the 3 fingers will be used for vibrato, then take the ring finger off and only 2 will be used for...