Vietnam’s leaders convene next week for a once-every-five-years congress, where General Secretary To Lam is looking to cement control over the Communist Party ruling one of Asia’s fastest-growing economies. The nation of 100 million people is both a repressive one-party state and a regional economic bright spot, with the party seeking to deliver rapid expansion to underpin its claim to legitimacy. But its leaders face challenges from tensions between its main trading partners, the United States...