Combine water and sugar in a large saucepan over medium heat. Scrape seeds from vanilla bean; add seeds and bean to pan. Bring mixture to a simmer, stirring constantly; cook 10 minutes. Add peaches to sugar mixture; simmer 5 minutes or just until tender. Remove peaches with a slotted spoon, and place in a bowl.
Bring sugar mixture to a boil over medium-high heat; cook until reduced to 1 cup (about 5 minutes). Remove and discard vanilla bean. Add blackberries; cook 10 minutes, stirring occasionally. Remove from heat; cool 5 minutes. Place berry mixture in a blender or food processor; process until smooth. Strain mixture through a fine sieve over a bowl; discard solids. Stir in juice.
Divide ice cream evenly among 6 bowls; top with peaches and blackberry sauce.