In a nonreactive medium saucepan, combine 1 cup of the blueberries with the sugar, vanilla bean and 3/4 cup of water. Bring to a boil over moderately high heat. Reduce the heat to moderately low and simmer until the berries start to fall apart, about 5 minutes. Discard the vanilla bean and transfer the mixture to a blender or food processor. Add the lemon juice and blend until smooth. Strain into a large bowl, add the peaches and plums and gently toss to combine. Sprinkle with the remaining blueberries. Serve chilled or at room temperature garnished with the mint.