Vassallo first joined the Green movement, a now defunct group, in 1979. 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 edited the movement's magazine "L-Ambjent" (1987) and served as its General Secretary (1989). Vassallo compiled and co-edited The Hielsa Human Rights Report 1984 which contributed in drawing the attention of the
Helsinki Watch to human rights violations in Malta and to electoral system reforms prior to the 1987 General Election. 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. Vassallo also stood on a Green Party ticket, this time successfully, in 1994 in the local election in
Sliema. He has served as Chairperson of DA] between 1998 and 2008. On the international scene Vassallo was Malta's delegate first to the
European Federation of Green Parties and then to its successor the
European Green Party (EGP) (1999–2006). He also served as a member of the EGP Committee between 2006 and 2008. ==European Commission==