In general, we usually recommend using a motor driver that has a continuous current rating greater than the stall current of your motor. From the motor specifications I found on this website, it seems like you should be able run two of those motors from each channel on the VNH5019.
I tested our Spring RC and Parallax continuous rotation servos over an update frequency range of 50Hz to 200Hz to see how they were affected. The only change in performance I noticed was that at slow speeds the servo moved a little bit faster with higher frequencies. However, the change was not a significant jump in performance, and changing the frequency had no effect on the top speed of the servos.