Thank you again for all of your input, everyone! I returned the Halogen vest, which I think is pretty nice, but nowhere as nice as the Cos vest. I also decided to order the Cos vest one size smaller, just to see how the fit looks. I like that the stiffer textile doesn't cling or drag anywhere.
As far as styling goes, I think the severity of the Cos vest calls for equally simple garments. That's why the white shirt and the jeans work so well with it. I think a patterned top might just come off as frumpy. I'm so delighted to be wearing that particular white shirt, which was fairly expensive, and has been a closet orphan for some time.