He was the only son of Manuel Caetano da Silva and his wife Maria da Encarnação Vassalo, and was the brother of the feminist author and anti-government campaigner Maria Lamas. He was married, with two daughters, Joana and Aurora.
In 1958, he was nominated to replace Paulo Bénard Guedes as the 128th Governor-General of the Portuguese State of India. At the same time, he was also appointed Commander-in-Chief of the Portuguese Armed Forces in India. When the Republic of India sought to annex the territories of Goa, Daman and Diu from Portuguese control in December 1961, Manuel Vassalo e Silva, recognizing the futility of facing a superior enemy, disobeyed direct orders from the Portuguese President of the Council of MinistersAntónio Salazar to fight to the death and surrendered the following day to the Indian invasion, following several losses and the destruction of a battleship. After that he fell into disgrace at the eyes of Salazar, who never accepted the fait accompli of the annexation. Vassalo e Silva was greeted with a hostile reception when he returned to Portugal. He was subsequently court martialed for failing to follow orders, expelled from the military and was sent into exile. His rank and freedom were restored only in 1974, after the fall of the regime, and he was given back his military status. He was later able to conduct a state visit to Goa, where he was given a warm reception. Recently the Goan-Portuguese politician Narana Coissoró said that Salazar sent him a cyanide capsule for use in case of defeat .
Family
He was married to Fernanda Pereira e Silva Monteiro and had a son and two daughters:
Fernando Manuel Pereira Monteiro Vassalo e Silva, married to Maria Amélia Franco Veiga, daughter of António Veiga and wife Rosa Maria Garcia Franco, and had issue, seven children, two married and had issue
Maria Fernanda Pereira Monteiro Vassalo e Silva, married to Rui António da Cunha Bernardino, and had issue, eight children, six married and had issue
Maria da Luz Pereira Monteiro Vassalo e Silva, married to António Faias Sors Lagrifa, born in Luanda, and had issue