Vatican Diplomatic Corps
The term Vatican Diplomatic Corps, if used to mean the Diplomatic Service of the Holy See, can be considered on two grounds a serious misnomer or at least gravely ambiguous.
The Holy See is distinct from the State of Vatican City, having existed long before the creation of the latter (in 1929) and being, in the eyes of the governments that have diplomatic relations with it, far more important than the 44-hectare statelet.
The term diplomatic corps properly means the whole body of diplomats, or at least the heads of mission, accredited to a government, not by a government.
If by "diplomatic service" is meant a body of representatives of a government permanently posted abroad to foster relations with the government of their residence, and not merely sent on particular missions, the diplomatic service of the Holy See is the oldest now in existence. The earliest was that of the Republic of Venice. Only under Henry VII of England (1456–1509) did that country begin to make use of permanent embassies (Hyginus Eugene Cardinale: The Holy See and the International Order, Colin Smythe 1976, ISBN 0-900675-60-8, p. 61).
If the apocrisiarii of the Pope to the government of the Byzantine Empire, who were resident in Constantinople, and the legati that he sent on important special missions are considered to be, in a broader sense, diplomats, then the history of the diplomatic service of the Holy See can be said to have its origins in a period before even the creation of any of the present-day European states.