OK I just quickly checked out my Strad (don't have the cello yet, but Bday is coming soon) and I assume you are talking about the other two knobs under the sliders.
One is the Vibrato Rate Offset and one is the PB Vibrato Inhibit Threshold. Now I have NO idea what they actually do, and since I'm a real man there is NO way I'm going to dig out the manual and look it up, but I will twist them around some and see how it effects the sound.
Vibrato Rate Offset is for setting the minimum vibrato rate. In other words, the vibrato rate for zero Aftertouch.
PB Vibrato Inhibit Threshold is for preventing vibrato on rapid pitchbends (PB). Lower values will inhibit vibrato on lesser pitchbends, while higher values will allow vibrato to persist.
thanks again..... And indeed for a violin there should have been 4 knobs.
As I understood well, that last one is for preventing vibrato with short pitch changes, with other words: when there is fast bowing (is the jazz term: fast riffs?)
Raymond - experimenting with knobs (uuhuhuhuhuh?!)