Shortly after the outbreak of Second World War in September 1939, Lieutenant General Sir Thomas Blamey appointed Vasey to the staff of the 6th Division. In March 1941, Vasey took command of 19th Infantry Brigade, which he led in the Battle of Greece and Battle of Crete. Returning to Australia in 1942, Vasey was promoted to major general and became Deputy Chief of the General Staff. In September 1942, he assumed command of the 7th Division, fighting the Japanese in the Kokoda Track campaign and the Battle of Buna-Gona. In 1943, he embarked on his second campaign in New Guinea, leading the 7th Division in the Landing at Nadzab and the subsequent Finisterre Range campaign.