I'm not sure if this is true but I've been told the Little Saigon has the most Vietnamese people outside of Vietnam. It's a great place to visit. Especially check it out on Tet day one of the major celebrations.
Directions: A good place to start is the area near the Dragon Phoenix Palace Chinese Seafood Restaurant. Little Saigon runs from Beach Blvd to Euclid along Garden Grove Blvd, Westminster Blvd and Bolsa Ave.