Originally Posted by ironchef
Regarding your question, it's because of the differences in viscocity and pH levels in both the cream base and the vinegar base. Acids weaken the thickening power of cornstarch, and this also works against the starch's chemical bond. The vinegar enabled the cornstarch to dissipate rather than clump.
Hey I learned something new! I love great info like this that helps one understand the whys in the kitchen. Thanks IC!