There’s a rule followed by savvy diners looking for the ultimate in ethnic authenticity: Eat where the locals eat. The search for authentic Vietnamese food in the United States will always end in one community, Little Saigon, in Southern California’s Orange County. Here lies a spectacular enclave, built by extraordinary immigrants who started with virtually nothing. Today, in an area of roughly 3 square miles, Little Saigon is home to more than 4,000 Vietnamese American businesses and 200 restaurants—and the largest population of Vietnamese outside Vietnam.