The best way to find out about Gore's lineage is to read his book Palimpsest and also an article from the 80s in House and Garden called "In Aunt Fenita's Footsteps."
Gore has done a lot of work on his own genealogy.Unfortunately, the Vidals lived in the very northern part of Italy (the Romansch area north of Venice) and they left it very, very long ago.His branch of the family comes through Austria (there is still a Vidalhaus in Feldkirch, Austria).
Here's what I have (since I actually am a cousin)
Eugene Luther "Gore" Vidal, born 03 Oct 1925 in West Point New York.
Son of Eugene Luther Vidal (1895-1969, born S. Dakota) and Nina Gore (1903-1978, born Lawton, OK).
Eugene Luther is the son of Felix L Vidal (b. 1862 in Wisconsin) and Margaret Ann Rewalt (b. 1870 York, PA) His brother was Felix Vidal, the noted aerospace pioneer and brigadier general.
Felix L. Vidal is the son of Eugen Fidel Vidal, born 1821 in Feldkirch, Austria.There were other children, including Fenita (the "Aunt Fenita" of the article), Fanny (my great-grandmother -- got her picture on my wall), Mary and Herminia.
Eugen Fidel Vidal married Emma Hartmann, Born 1830 in Lucerne, Switzerland.
Eugen's parents are Johan Felix Vidal, born in the Friuli at Forni a. Voltri, Italy, and some "burgermeister's daughter."
Almost all of this is from Gore's own work.The family can also be tracked through Wisconsin with the elaborate census info on Ancestry.com.