Traction control and VSC are two seperate safety features. Putting the IS-F in sport mode brings up the limits on what you can push the car to, plus it enhances the engine response, braking, and transmission.
Turning the VSC off still gives you the engine, trans, and braking "performance", but this time without the traction control and Vehicle Stablilty Control.
Look at it this way, in Sport mode, it's like letting you go balls out until the vehicle notices it needs to step in to correct your mistake of either braking too late, turning too fast, etc etc....
Being in Sport Mode with the VSC is balls out, you make a mistake, you better recover on your own, because you just bypassed the Vehicle Stability System and traction control..
Hope that helps..