St. Louis, with the squadron, reached Pearl Harbor on 6 August, refueled, and sailed on the 8th for Vietnam. On 16 August, she was detached to proceed to Subic Bay and finally rejoined her squadron at Danang on 21 August. After offloading Marines and their equipment, she then proceeded to Buckner Bay, Okinawa, returned to Long Beach to transport a World War IImidget Japanese submarine to the submarine base at Pearl Harbor; and anchored again in Danang Harbor on 11 October. After completion of a large redeployment operation involving over 2,000 Marines and 22,000 tons of equipment in the Quang Nam province, St. Louis visited Hong Kong and then moved to Subic Bay in thePhilippines to participate in large scale amphibious landing exercises during November and December.
St. Louiscompleted the amphibious exercise in early January, spent 15 days in upkeep in Subic Bay, then headed north again for two months of shuttling men and cargo between Vietnam, Okinawa, and Japan. She departed from Yokosuka on 20 March 1971 and entered Long Beach on the 31st. After a month and a half stand down period in Long Beach and three more weeks of local operations and upkeep there, she returned to Vietnam, arriving in Danang on 24 June. She visited Hong Kong, 28 June to 3 July, then returned to Long Beach on 19 July. St. Louis remained on the west coast for the remainder of 1971 and for the first three months of 1972. During this period, she was engaged in refresher training, amphibious exercises, and upkeep.