Not sure of the official slim and save line on this, but a quick Google shows that it IS allowed on some other VLCD's - so I'd say give it a whirl but if it makes you hungry, or triggers cravings, then stop having it.
I've checked up on this and apparently Stevia (a much healthier alternative to other sweeteners) is fine to use as long as you use the liquid stevia. Apparently, the powdered stevia has maltodextrin which is basically the same as a carb. Hope that clears things up