Vanderbuilt is Mr. Gronkle's nephew, but it is unknown which parent's brother Mr. Gronkle is. He appears in The Busy World of Richard Scarry and Busytown Mysteries, both. In the former of the two, he is voiced by Michael Caloz; in the latter, he is voiced by an unknown woman.
Vanderbuilt is gray in The Busy World of Richard Scarry, but he is brown in Busytown Mysteries. Because of the latter, it is not apparent that he is Mr. Gronkle's nephew or otherwise related.