The vdc does out perform the old stock LSD for that very reason. Plus in the rear it has a more positive power distribution then the LSD could ever have. In essence the VDC can simulate a vehicle having locked differentials in the front and rear.
Take a vehicle with locked diffs and put two opposite corner wheels in the air. Due to the locked diffs the wheels on the ground still have 100% power.
Same situation with the older rear LSD like in 2007 Forester. The front wheel in the air will spin wildly as it is the path of least resistance. The rear wheel on the ground will only get a small amount of power as the center viscous coupler only helps to equalize the power distribution, it still favors the path of least resistance(the front). Now the rear LSD works on the same principal as the center viscous coupler. So it in turn sends the majority of the power to the rear wheel in the air. Which in the end equals you going no where.
Again same situation but with the VDC. The VDC sees the wheels in the air spinning wildly and the wheels on the ground not turning(does so via the ABS wheel sensors). It then starts to apply the brakes at those two wheels, until their speed matches that of the wheels on the ground,never fully locking them up. Simulating the function of the locked diffs with out a locker.