The modern Key Vaca is located on U.S. Highway 1, at approximately mile markers 47.5--53, between Fat Deer Key and Knight's Key.

Testimony given in 1677 and preserved in the Archive of the Indies in Seville, Spain uses the name Cayo de Bacas. The Spanish shipwreck chart of 1733 shows Cayo de Baca. Juan de Liguera chart (1742) shows Cayo de Bacas. The Father Alaña chart (1743) shows Bacas. William Roberts, in his First Discovery and Natural History of Florida (1763) uses Cayos de Vacas. The O'Caroll chart (1770) shows Cayo de Bacas. DeBrahm chart (1772) shows Vacas Islands. Bernard Romans chart (1774) shows Cayos Vacas. Gauld chart (1775) shows Cayos de Vacas or Cow Kays. A French chart drawn by Chaulaire in 1800 shows I. de Vache. Blunt Chart (1846) shows Cayos de Vacas or Cow Keys. Gerdes, in his Reconnaissance (1849) states " The large island W. of the Grassy Ids. and E. of the Cow Harbor is called the Vaca or Cow Key." The 1868 U.S. Coast Survey chart shows Key Vaccas.

The history of place-names in the Florida Keys reveals that Key Vaca was named at a very early date. Like Matecumbe, the name Vaca originally signified not a single island, but a group of islands. In an article by Florence Brigham in the 1957 issue of Tequesta, Key Vaca is said to have been known by its place-name before the founding of Saint Augustine in 1565. Brigham does not provide her source for this data. The largest portion of the city of Marathon is located on Key Vaca. A 1935-36 field report written for the U.S. Coast & Geodetic Survey states: "The spelling of the name of this key is uncertain. All local authorities agree that a final 's' should be added and this checks the local pronounciation. Opinion is divided as to whether the key was named "Vaccas" by the original owner [a Spanish friar named Ferrera for a friend of his, or "Vacas" [Spanish for cattle] for the cattle which local legend has grazing on the key at a distant date. Authority 1 (Charles L. Pinder of Key West) who must have had some information from the inhabitants of Conchtown on the Southeast section of the key gives "Vaccas" and this name is recommended."