Measure rice, water, and salt into medium saucepan. Bring to a boil; reduce heat to simmer and cook uncovered for 40 minutes or until water is absorbed.
Place rice in top of a double boiler; stir in sweetened condensed milk, margarine and raisins, if desired. Cook in double broiler until mixture has thickened, stirring occasionally, about 20 minutes. Remove from heat; stir in vanilla.