Jan 29 (Reuters) - Tension with China about competing claims over portions of the South China Sea remains the top political risk in Vietnam, capable of creating international and domestic rifts. Major arms purchases may soon be under discussion, even as economic growth slows. Late last year Prime Minister Nguyen Tan Dung offered a rare apology for Vietnam's economic problems, and said the ruling Communist Party should work to implement reforms more quickly. RATINGS: (Unchanged unless stated)