Vehicle Stability Control works differently between the different manufacturers, but usually, sensors sense the intended direction (through brake use, steering input and throttle input) and the actual movements of the vehicle (through wheel speed sensors, yaw sensors, etc), through complex logarithms, the computer will then brake individual wheels and/or reduce throttle input in an attempt to bring the vehicle onto the intended course.
The VSC is not intended to reduce a rollover, and in certainty, cannot. Ford has a new system, call something like "Roll Over Control" or some such which acts like a more aggressive VSC to predict a pending rollover, take action to prevent it, and in the event that wheels leave the ground, override nearly all driver inputs to right the vehicle again.
All of these systems are designed to work ON ROAD, and in normal circumstances, so if you leave the pavement the systems can do very little to help you. So to answer the question, no they will not help prevent a rollover if you hit a tree or a wall, no automated system can help (outside of airbags) once an accident occurs, mainly as all these electronic nannies are in place to prevent accidents, not function through and after an accident.
