Combine first 4 ingredients in a large saucepan; stir well. Stir in rhubarb and strawberries. Bring to a boil over medium-high heat, stirring often. Reduce heat, and simmer, uncovered, 5 minutes or until sauce is thickened and fruit is tender, stirring often. Remove from heat, and let cool; stir in vanilla.
Serve sauce over low-fat frozen yogurt, angel food cake, or fresh fruit. Store sauce covered in the refrigerator for up to 1 week.