In 1981 he became a member of Żgħażagħ għall-Ambjent, one of the first Maltese environmentalist movements, which was later to rename itself to Moviment għall-Ambjent and, eventually, Friends of the Earth (Malta).
Vassallo was one of the co-founders of Democratic Alternative (DA) in 1989 and unsuccessfully stood for all general elections since on the Party ticket.