Vassallo has worked with the European External Action Service for more than 11 years. Before serving as the Deputy Head of Delegation, Vassallo worked as a political counsellor with the EU Delegation in Turkey and in the Philippines. Prior to this, Vassallo served as Head of European Parliament Office in Malta, Member of the Policy Planning Unit for the Council of the European Union, Foreign Relations Counsellor with the Permanent Representation of Malta to the European Union, First Secretary with the UN Maltese Permanent Representation, and Second Secretary at the Maltese Ministry of Foreign Affairs.