I've never personally programmed Vex, but I'm told that it's pretty much the same as the FRC controller, so they might be interchangeable.
As for documentation, Kevin Watson includes pretty extensive readmes with all of the code he writes (the C18 3.0+ beta not yet included), as well as pretty extensive documentation for C18 and the RC. All of this can be found on his code repository (kevin.org/frc).
If you have specific questions about his code, he patrols these forums pretty frequently. You can either wait for a response here (I wouldn't be surprised if he did), post in one of his threads or email him. One way or another, I'm sure you'll get an answer.
As for how close the default code gets you to a working robot, you can get basic drive code in a couple of lines, e.g.
Code:
pwm01 = p1_y
pwm02 = ~p2_y
This assumes that your motors are hooked up to pwm outs 1 and 2, your joysticks are plugged into ports 1 and 2, and that whichever motor needs to be flipped due to position is in pwm out 2. Hopefully Kevin's documentation or programmers better at this than me can shed some mroe light on this for you.