His masterpiece is the basilica of Ta' Pinu in Gozo, while other notable works include the domes of the Ħamrun and Siġġiewi parish churches, Villa Rosa and the now-demolished Casa Said.

[3] Vassallo was awarded the Order of St. Gregory the Great by Pope Pius X for his services during the Eucharistic Congress of 1913, for which he had designed a tribune.

[3] Vassallo designed or was involved in the construction of many buildings and structures, including hospitals, schools, workshops and houses, and he used various architectural styles throughout his career.

[3][6] In the 1920s, Vassallo won a competition for the design of the dome of the Parish Church of St. Cajetan in Ħamrun, which would eventually be built in 1953–55 under the direction of Ġużè Damato.

Vassallo also designed the dome of the Church of St. Nicholas in Siġġiewi (1919), the Rococo Revival Sanctuary of Our Lady of Tal-Ħerba in Birkirkara (1923), and the Neo-Romanesque basilica of Ta' Pinu in Gozo.