There exists a couple of short survey articles which provide the researcher with a basic overview of the city state’s legal system. The most useful of these is Jorri Duursma, “The State of the Vatican City,” in Fragmentation and the International Relations of Micro-States. Although now a little dated, Duursma’s 45-page chapter provides an excellent overview of the history and status of the VCS Particular attention is paid to the international status of the city state and the extent to which it has achieved independence from Italy.
Also recommended, albeit far more concise and slightly more recent, is Robert Shelledy’s "Vatican" entry in Legal Systems of the World. This succinct entry provides an historical account of the development of the legal system, together with a description of the legal concepts that govern the nation and an accurate depiction of the current structure of the legal system.