You can try resistors, or diodes. One method mentioned a while ago was to add two diodes in line witheach motor. That would knock the voltage down a bit, you can even add 4 if necessary. Remember the motors are reversable so the diodes need to be inparallel (anode to cathode) so current can flow in both directions. For resistors either place a low resistance resistor in series with the motors or place a high wattage one in parallel to steal some of the power away from the motors (although it will get hot & waste power).

I don't know what voltage levels are usedto drivethe VS motors, I don't have one opened up right now. For the diodes my guess is you can use ones with 1 amp rating or better. The "PIV" of the diode is not a concern, these are operating on low voltages. The resistors would need to be higher wattage, not sure about the values but they would be low, maybe a couple of ohms.