After World War II, he covered several posts in the state administration before becoming the head of the City Historical Archives of Ljubljana. In 1961 he spent 6 months in Paris studying economic history with Jean Meuvret at the École pratique des hautes études. In 1971, he became a professor of legal history at the Faculty of Law of the University of Ljubljana. Vilfan eventually became one of the most renowned experts for the legal and economic history of the Slovene Lands. He also wrote about early medieval settlement patterns.
Sergij Vilfan was a member of Académie européenne d'histoire in Brussels (from 1981), a member of Société Jean Bodin (from 1974), a member and president of The International Commission for the History of Towns (ICHT) and collaborated with the International Commission for the History of Representative and Parliamentary Institutions (ICHRPI). For his work, he received several prizes at home and abroad, among them, in 1980, Anton-Gindely Preis from the Institut für den Donauraum und Mitteleuropa in Vienna (Austria).
He died in Ljubljana in 1996.
Major works
Pravna zgodovina Slovencev ("Legal History of Slovenes". Ljubljana: Slovenska matica, 1961).
Rechtsgeschichte der Slowenen bis zum Jahre 1941. Graz: Leykam Verlag, 1968.
Die deutsche Kolonisation nordöstlich der oberen Adria und ihre sozialgeschichten Grundlagen ("The German Colonisation of the North-Eastern Part of the Upper Adriatic and its Social-historical Basis". Sigmaringen: Jan Thorbecke, 1974).
Wirtschaftsgeschichte und Rechtsgeschichte ("Economic History and Legal History. Graz: University of Graz, 1985).
Les communautés rurales entre l'occident et les Balkans : formes Slovenes jusqu'au début du XXe siecle ("Rural Communities between the West and the Balkans: Slovene Formes until Early 20th Century". Paris: Dessain et Tolra, 1986).
L'individu face au pouvoir à la fin du moyen age : essai de comparaison entre l'Empire serbe et les provinces alpines du Sud-Est ("The Individual and Power in Late Middle Ages: A Comparative Essay between the Serbian Empire and the Southeastern Alpine Areas". Brussels: Editions de la libraire encyclopédique, 1989).
Crown, Estates and the Financing of Defence in Inner Austria, 1500-1630 (London: Macmillan & School of Slavonic and East European Studies University of London, 1991)
Towns and States at the Juncture of the Alps, the Adriatic, and Pannonia (Boulder, San Francisco, Oxford: Westview Press, 1994).