Whoa, way to revive an old thread!
The key part is a variable voltage regulator (probably an
LM317T), so take a look at the
datasheet to find out exactly what resistors you need. The Vex controller needs between 7.2 and 9V. You could also use a
straight 9V voltage regulator, although you would probably have to order these instead of getting them from Radio Shack.