well, I received my order and had some time to experiment,
so I thought I'd pst my findings.
The the only difference between the vellum card stock and paper is just the weight - the card stock is definitely heavier. Still translucent (although not as much as the paper, of course) but definitely thicker. And when I ran a pice of it through my Sizzix machine with the Swirls embossing folder, it came out great! Not one tear. A few stress marks (areas that are a little white and that look like maybe they came *close* to tearing) but absolutely no tears. Certainly nothing like what I experienced with the vellum paper!
So I'd say that the only real difference between vellum card stock and the paper is the weight and that the card stock is definitely preferred when embossing with the CB folders. Hope this was helpful.