Grease a 6-qt. oval slow cooker with 1 tablespoon softened butter. Toss peaches with brown sugar, lemon juice, vanilla and, if desired, rum; spread evenly in slow cooker., Whisk together first 6 topping ingredients; stir in oats. Cut in butter until crumbly; sprinkle over peaches. Cook, covered, on low until peaches are tender, 3-4 hours. If desired, serve with whipped cream.