Born in Heidelberg, Vossler was the younger son of the Romanist Karl Vossler and his wife Ester Countess Gnoli, daughter of the Italian poet and literary historian Count Domenico Gnoli.
Growing up in the bilingual parental home, Vossler could translate and publish after the death of his father his correspondence with the Italian anti-fascist Benedetto Croce.
He completed his studies at the Ludwig-Maximilians-Universität München in 1925 under Hermann Oncken with a doctorate on Mazzini's political thinking in the intellectual currents of his time.
In 1929 he habilitated, also under Oncken, at the Humboldt University of Berlin with a study on the American revolutionary ideals under Thomas Jefferson.
In addition to the revolutionary movements of the 18th and 19th centuries, his research on Jean-Jacques Rousseau, Alexis de Tocqueville and the development of national thought were also important.