Vice President Kamala Harris will go to Vietnam next week with a message to its military: We’ve got your back in the South China Sea. It is a hugely important waterway — more than one-third of all global trade flows through it. And China says pretty much all of the South China Sea belongs to China. Vietnam is one of several countries with claims over the same sea. Vietnam isn’t just up against China’s navy. It’s also facing a mysterious Chinese fleet that, according to Beijing, does not even exist. The World’s Patrick Winn reports.