BY JEREMY PAGE

BEIJING—China Vice President Xi Jinping, the man expected to take over as China's top leader next year, will face one of the first serious tests of his diplomatic acumen next week when he visits Vietnam, which is now forging closer ties with the U.S. due to an increasingly tense territorial dispute with Beijing.

Mr. Xi will also go to Thailand, the Chinese Foreign Ministry said Friday, while top Chinese foreign-policy official Dai Bingguo will visit Myanmar in what diplomats and analysts see as an attempt by Beijing to improve ties with its neighbors following Washington's unveiling last month of a ...