As I understand it, there are (at least) two different ratings - SB% and running speed. My conception of it is that SB% directly and accurately reflects how good of a basestealer you are. The game is replete with fast guys who can't steal as well as you might expect (Bernie Williams being the most notable) and also with slow guys who can (Ryan Klesko, Mike Sweeney).

In my view, the speed rating should reflect top speed which has a big impact in the game (i.e. first to third, second to home, etc.). Vlad is a good but not great basestealer (20 CS makes this clear). He is also one of the fastest guys in baseball (the fact that he was able to steal 40 despite having bad basestealing skills is great evidence of this).

As an example of this, Mike Cameron's speed rating is 8.1, while Vlad's is 6.8. I have seen both of these guys play. Vlad is faster. By quite a bit.