1. Place whole apples and pears in a large heat-resistant bowl, and cover with boiling water. Let stand 10 minutes, turning fruit a few times. Drain; peel fruit, and cut into chunks.
2. Stir together apples, pears, sugar, 2 Tbsp. lemon juice, and cinnamon in a 3 1/2-qt. saucepan. Bring to a boil over high heat, stirring often. Cover, reduce heat to low, and simmer, stirring occasionally, 8 minutes or until tender.
3. Gently stir in plums, and cook 2 to 3 minutes or until plums are tender. Remove from heat; stir in vanilla, and, if desired, an additional 1 Tbsp. lemon juice. Serve warm or cold.
Prepare this up to a day ahead, and store, covered, in the refrigerator. The amount of lemon juice you use will depend on the sweetness of the fruit.