Vassallo studied the works of his compatriots Sinibaldo Scorza (1589-1631) and Giovanni Benedetto Castiglione (1609-1664), whose respective studios he probably studied in. Scorza was in Genoa from 1627 to 1631, while Castiglione was in Genoa from 1850 and did not leave for Rome until 1632. Under the influence of De Scorza and Castiglione, as well as his teacher Malò, Vassallo would have learned to excel in realistic detail by combining animal and figure representations in his works.
Vassallo continued to receive important public commissions until the end of his career, such as the Martyrdom of Saint Marcello Mastrilli for the Convento di Carignano (now in a private collection). Soprani also mentioned that Vassallo painted a large number of portraits.
In addition to mythological and pastoral scenes, he also painted a growing number of still lifes throughout his career.
The circumstances of Vassallo's premature death are not really known. According to Soprani, the seriously ill artist was encouraged to go to Milan for its beneficial climate. He died there between 1664 and 1673 without having any descendants. He had no known pupils to whom to pass on his artistic legacy. The closest heir to Vassallo's style is Giovanni Agostino Cassana (c.1658-1720); many of his still lifes repeat the same subjects and motifs of his master.