"Commonitory" is correct. I couldn't find a definition of it on the web, but it clearly is the word used for St. Vincent's text. Knowing the little that I do about St Vincent's teachings, I would assume that the purpose of the book is to outline that which is common
to the Catholic church. (Not Roman
Catholic. St Vincent lived long before the Great Schism.)
Edit to add: it is available online at http://www.ccel.org/ccel/schaff/npnf211.toc.html
, though I can understand why you would want a hard copy.