KIEN VANG, Vietnam — From an American gunboat decades ago, John Kerry patrolled for communist insurgents along the winding muddy waters of the Mekong Delta. From those familiar waterways that eventually turned the young lieutenant against the war, the top U.S. diplomat confronted a modern enemy Sunday — climate change. Connect With Us on Twitter Follow @nytimesworld for international breaking news and headlines. Twitter List: Reporters and Editors In this remote part of southern Vietnam, rising sea waters, erosion and the impact of upstream dam development on the Mekong River are proving a more serious threat than the Viet Cong guerrillas whom Kerry battled in 1968 and 1969. “Decades ago on…