WHEN Prime Minister Phan Van Khai of Vietnam wraps up his trip to the United States today , he will have gathered with President Bush at the White House and Bill Gates of Microsoft in Seattle , rung the bell to open the New York Stock Exchange and met the presidents of Harvard and M.I.T . But two stops left off his itinerary were Los Angeles and Silicon Valley , home to the biggest communities of Vietnamese-Americans and epicenter of anti-Hanoi sentiment .