I could swear this shows how to do it (for simple "objects", with "proper" setup):
http://nootropicdesign.com/projectlab/2011/03/20/arduino-computer-vision/
Basically AWOL is right - you just need to make sure the ball is the brightest thing; this would likely mean either painting (as suggested) or illuminating the ball, and tracking its motion against a dark background. Unfortunately, I doubt the Arduino/VE combo will be fast enough to do that to track the tennis ball at normal speeds; you didn't say how fast the ball was going to move, nor the environment it would be in - but if you are trying to track a tennis ball being batted back and forth on a tennis court by players in varying lighting conditions, then an Arduino/VE is not going to do it (it won't have the speed/frame-rate, nor the resolution needed - among other things).