There's lots of his logos throughout. These also had gold on them at one time. These are individual slate tiles, tied into the roof which we got to see when we were in the attic.

From Bob Vila: Roof Tiles and Copper Flashing
The slate roof tiles were affixed one by one. Each piece was drilled at the corners and subsequently wired onto the attic’s steel infrastructure. Copper flashing was then installed at the junctions to prevent water from penetrating. The fanciful flashing on the ridge of the roof was embossed with George Vanderbilt’s initials and other natural motifs from his family crest. The original gold leaf no longer survives.