By Matt Spetalnick and Mai Nguyen HANOI (Reuters) - U.S. President Barack Obama arrived in Vietnam late Sunday ahead of a three-day trip aimed at sealing the transformation of an old enemy into a new partner to help counter China's growing assertiveness in the region. Four decades after a war with Vietnam that deeply divided opinion in America, Obama aims to boost defense and economic ties with the country's communist rulers while also prodding them on human rights, aides say.
