China’s leader, Xi Jinping, arrived in Vietnam on Tuesday for a relatively rare trip abroad, seeking to elevate ties with an important neighbor just three months after President Biden visited Hanoi on a similar mission. Few nations now feature more centrally in the great-power competition between the United States and China, placing Vietnam, which has a long history of fierce independence, in a high-risk, high-reward position. Keeping both giants happy could mean a transformative economic boost; angering one or the other could bring heavy costs. “This is a very delicate…