Boston — All the fifth-graders in the classroom of Phan Hu ngoc Chan open their lesson books to the play ''Mr. Hare,'' and, judging from their waving hands, most want to read the part of the hare. Pointing to each student, Miss Phan assigns the roles: hare, tortoise, squirrel, badger, skunk, beaver. Poised on the edge of his seat, the Vietnamese boy with the coveted role of the hare begins reading aloud in careful but well-spoken English. ''These students are already reading at a fifth-grade level,'' whispers Miss Phan, their teacher.Almost 200 Vietnamese students are attending kindergarten through fifth grade in the bilingual program at the Jackson-Mann Elementary School in Brighton. Here the children, most of whom have been in the United States less than three years, learn English as well as the math and social studies lessons taught to American students at the school.Vietnamese students from across the city spend three years in the bilingual program here, gradually becoming ''mainstreamed'' into the regular academic program, says principal Gregory Toupouzis. For adults, the transition to English is more difficult. Many attend classes offered by private voluntary agencies such as the International Institute of Boston. Others, who cannot or will not come to the classes, are taught in their homes by community volunteers.Cindy Rodgers, an employee of the Massachusetts Department of Public Health, is one of more than 200 volunteers trained by the International Institute to work in the refugee community. Since June, she has been meeting once a week with a group of Cambodian refugees. While they have been learning English, Ms. Rodgers says, she has had ''a glimpse of what it's like to enter this country and this culture.'' The experience ''has deepened my understanding about what their lives were like. They talk about their homes, and the people they left behind,'' she adds.Although progress has been slow, Ms. Rodgers says she's now beginning to see improvement in the refugees' language skills. ''It takes time - and trust.''