I know it’s suppose to stop rollover, but what is it able to do to accomplish this?
What does VDC actually do?
My understanding based on past reading is that is cleverly applies the brakes on a wheel by wheel or axle by able basis to bring your car out of understeer and oversteer situations.
Google turned up a concise explanation like you probably wanted:
"Understeering
If you understeer, the front end of the car tends to slide out. In this situation, ESC would apply the antilock brakes to the inside, rear. It may also reduce the engine’s power.
Oversteering
If you oversteer, the rear end of the car tens to slide out, otherwise known as a ‘fishtail.’ In this case, ESC would apply the outside, front brake to correct. "
Cite: http://autoadvice.about.com/od/safety/a/ESC.htm
I found a list of some vehicles that use it here:
http://www.safercar.gov/pages/ESC-EquippedVehicles-2005.htm
::: Waves hand::: I know! I know!
I teach this stuff.
for the systems found on the cars that I teach on (Volvo) the full on all speed dynamic control is referred to as DSTC Dynamic Stability and Traction Control. There is a sensor cluster under the front passenger seat that reads yaw, and lateral G forces. On the steering shaft is a steering angle sensor that reads steering wheel position with a resolution of 4 degrees.
The system is always active whenever the speed of the car is over about 3 mph. When the driver turns the wheel, that input is sent to a control unit, which also monitors the yaw change and the g force change. As long as all of these inputs agree, the system does nothing. If however the system registers an out of line condition then the system will then act to make the car go where the driver has pointed the front wheels.
Let’s define some terms.
Understeer = the car wanting to continue in a straight line, and not follow the curve of the road. In racing sometimes called push, or tight. All street cars understeer, to a greater or lesser degree. Slight understeer is very easy to counter for any driver (even a novice) they turn the wheel, the car does turn quite as far as it should, then without even thinking the driver will turn the wheel a little bit more to compensate. However severe understeer is hard for a driver who has not been trained to counter.
Oversteer = The rear of the car wants to turn much more that the front of the car. In racing referred to as loose, an excellent description IMHO. This condition is very difficult for an untrained driver to handle.
So how does it work?
Anyway in the case of understeer, the system will reduce the throttle angle to reduce power (a big cause of understeer) and apply the brakes to the inside front wheel, and sometime the inside rear wheel to force a weight transfer to the outside front wheel, to increase traction on that wheel, and causing the car to turn in.
In the case of oversteer, the outside front wheel has too much traction, so the system brakes the outside front wheel, causing it to lose traction and track closer to straight ahead reducing the oversteer.
In addition If the driver steps on the brakes, the system will measure the brake pressure and adjust the response accordingly.
How well does it work? Freakin fantastic. The computer reads all the sensors and makes adjustments every 7ms. That is way faster than the speed of human control. The computer can control the car way faster than you or I. The first time I really tried it out was 1999, I was in Portland and it was raining. There was an offramp that made a 180 degree turn, and was posted at 15 mph. I entered the off ramp at 45mph, turned the wheel, and floored the gas. The car made some very interesting Berapp! noises and before I knew it I was around the corner, and headed toward the stop sign. There is no way in hell I could driven that corner at that speed. I do not posses that level of skill.
With that said, may I add a big YMMV Not all systems are this good, and some lower end cars have again IMHO some pretty crappy systems.
On our SUV (XC90) the system is augmented with a roll sensor that detects a potential roll over and if that happens the outside front wheel is locked momentarily and that causes the car to set back down on all four wheels. This of course will not prevent a tripped roll over, such as if you slide into a curb.
I hope you took notes, there will now be a quiz on the subject. 
Rick,
Thanks for all that info. Fascinating.
When you came out of that corner you mentioned, how fast were you going?
Sorry Mr. Slant I missed your question.
I exited the corner at just about the same speed as I entered. The braking the system did, was to regain / maintain control not to slow the car. Had I lifted off the gas, and hit the brakes, I would have exited at a lower speed.