I suspect that the head piece worn for the ball (as per the photo Mauriz posted) is a tiara set off centre on her head band. I don't have any other photos of this jewel. As you noted Alejandro E it is not the one we are accustomed to seeing. Sadly, for us Grace Vanderbilt seems to have eluded photographers and, as she rarely appeared in public in evening jewels, photos of her jewels are scarce. I suspect that, for the photos taken by Beaton, Grace Vanderbilt had a stomacher on her head band.
She seems to have been partial to wearing her stomachers attached to her head bands, a fashion she maintained long after it had gone out of fashion.
One jewel as a stomacher
and worn on a head band