In a small saucepan stir together the sugar and gelatin; stir in milk. Cook and stir over low heat until gelatin is dissolved. Remove from heat; stir in yogurt and vanilla.
In a blender container combine plum pieces and gelatin mixture. Cover and blend until nearly smooth. Pour the plum mixture into a 9x9x2-inch pan. Cover and freeze about 4 hours or until almost firm.
Break the frozen plum mixture into chunks. Transfer the chunks to a large, chilled mixing bowl. Beat with an electric mixer on medium speed until smooth but not melted. Return quickly to the cold pan. Cover and freeze about 3 hours or until firm.
Before serving, let frozen yogurt stand at room temperature for 10 to 15 minutes to soften. Makes 6 to 8 servings.
Prepare and freeze for up to 1 week.