In AD 1000 he was sent to Constantinople to work out the details of a plan his father and the Byzantine Emperor Basil II had been working on. Under this agreement the Venetians would conqueor Dalmatia and then hold it as a protectorate under Byzantine suzerainty. (Norwich, John Julius. "Byzantium: The Apogee" (Alfred A. Knopf: New York, 1992) p. 257).
While in Constantinople he was given a standard to use in his upcoming invasion of Dalmatia by the Bishop of Olivolo. He was successful in establishing Venetian power on the Dalmatian coast.
In 1004 Giovanni married Maria Argyra, who was the daughter of Basil II's second cousin. By this time Giovanni had been advanced to the rank of co-doge of Venice.
In 1006 Giovanni, along with his wife and son died of plague.