A vademecum is an authoritative handbook, a reference standard by which Catholics may know the truths of their Faith.
For example, St. John Paul II wrote a letter to Cardinal Ratzinger on February 2, 2003, in which he proposed a new Compendium of the Catechism of the Catholic Church. The Holy Father’s letter included this paragraph:
“The Compendium of the Catechism of the Catholic Church should contain, in a concise form, the essential, fundamental content of the faith of the Church whose completeness and doctrinal integrity it must fully respect so as to be a vademecum (handbook) that will allow persons, believing or not, to embrace in a general way, the entire panorama of the Catholic faith.”
Cardinal Burke also used the word vademecum in his Commentary on the General Directory for Catechesis and in Re-Christianize America.