I have leeches on my string but don't recommend putting them on the cables. The cables will move but won't generate nearly as much vibration as your string. If you think about it the string moves up or down and slightly to the rear of the bow. The string on the other hand is drawn back and the released forward violently and stops abruptly generating quite a bit of vibration.
If you're looking for more vibration reduction you may want to look at limb savers or cable guard dampener or something like the extreme limb saver which bolts directly onto the limb using the limb bolt.
http://www.limbsaver.com/limbsaver/a...stealthcd.aspx
http://www.limbsaver.com/limbsaver/a.../xtremels.aspx
There are ways to reduce vibration but scilencers on you cables isn;t the best way to accomplish it. IMO