A Los Angeles doctor who is fighting to save the life of a young Vietnamese refugee in the United States returned from Vietnam with blood samples that will show whether the refugee’s brother or sister can donate vital bone marrow. Thoi Le, 17, who came to the United States three years ago after fleeing Vietnam in a boat, suffers from aplastic anemia, in which the bone marrow stops manufacturing the white blood cells needed to fight disease. Doctors say a bone marrow transplant would give Le a 75% chance of regaining his health. But his parents and four brothers and sisters in the United States have been found to be incompatible as donors. Dr. Vasili Berdoukas, a researcher at Childrens Hospital in Los Angeles, said that if the blood samples are positive, he plans to bring the siblings to the United States.