Oh I didn't really know Gore Vidal's ancestors resided in Trento (North-Eastern Italy) nor that he might have a Sephardic Jewish origin.
Both of these two pieces of information sound very probable if you put them together, since in Trentino region lived an important Jewish community. Plus you can find further information on the Jewish origin of Vidal/Vitale last name in http://www.ancestry.fr/facts/Vitale-family-history.ashx
. It was a regular practice within the Jewish community to adapt their names and last names to the ones of the country they lived in: in Italy, for example, last names such as Benedetto, De Angelis, Leone, Pace, Vitale, etc do also belong to Jewish-Italian families and they usually come from a translation of a Jewish name: for example, Jewish-Italian last name Pace is a translation of Jewish Shelomo or Shlomo (Solomon in English), literally meaning 'peace' ('pace' in Italian).