He was born in Kristiania as a son of engineer Alf Vetlesen (1878–1963) and teacher Aagot Bugge (1881–1967). He was a great-grandson of Frederik Moltke Bugge. He finished his secondary education at Oslo Cathedral School in 1939, and then went to sea. He was going to return to land after one year, but when Norway was involved in World War II in April 1940, their merchant fleet became a crucial asset in the war and Vetlesen stayed. He worked at sea for several years, but while hospitalized with jaundice in Cardiff, he became involved with the British Communist Party. As he became known as a good speaker, he was hired in the trade union Norwegian Seafarers' Union in 1944.