"Cowboys- Vaqueros," Origins of the First American Cowboys, by Donald Gilbert y Chavez, contains a section (Chapter 11) on the language, or lingo, of the Cowboy, and is especially good with regard to those words which have been borrowed and/or altered from the original Spanish.
Buckaroo, indeed, is derived from Vaquero.

Available in printed form from the University of New Mexico, but also on line as a pdf book at this website:
http://www.nmhcpl.org/COWBOY.html

If you also are interested in New Mexico Hispanic Culture, go to http://www.nmhcpl.org
(click on Vaqueros to find the book)