BY JAMES HOOKWAY

CAM RANH BAY, Vietnam—America is back in Vietnam with a message for the rest of Asia: If you want to have a productive relationship with China, you'd better have a good one with Washington, too.

As President Barack Obama attends a key trade forum in Indonesia this weekend, the U.S. is moving beyond its decadelong focus on the Middle East to concentrate again on the powerhouse economies of East Asia at a time when the biggest success story of them all—China's rise to become the world's second-largest economy—is transforming the entire Asia-Pacific region.

Once America's fierce war-time adversary, Vietnam is ...