Because I'm am very much against using sweeteners like Splenda, Equal, Sweet-n-Low, etc. I'm using Stevia for this. For those interested, the best brand that I found--which only has stevia and natural inulin fiber in it--is SweetLeaf, which can be found at Whole Foods.
Whereas I'm also concerned about consuming WAY too much soy with this diet--too many conflicting issues regarding the consumption of soy (i.e. higher cancer rates)--I've replaced unsweetened vanilla soy milk with unsweetened vanilla almond milk. It has only 40 calories per cup, very low sugar, and has just as much of the "good stuff" in it as the fortified soy milk. The brand I love and use is Trader Joe's, but there are other brands out that that can be found either in conventional stores and at Whole Foods.
Hope this helps!