Prep: 10 minutes; Cook: 30 minutes, plus freeze time.
Granita: Scrape vanilla bean seeds into bowl. Add sugar; mash with fork.
In a small saucepan, warm 1/4 cup pineapple juice and sugar mixture until sugar is melted. Cool.
Combine mixture with remaining juices; freeze overnight in shallow pan. Remove from freezer 5–10 minutes before serving. Rake with fork to serve.
Meringues: In a medium bowl, whip egg whites, gradually adding 6 tablespoons sugar; beat until stiff peaks form. Fold in 3 tablespoons powdered sugar with spatula.
On a parchment-lined baking sheet, spoon meringue into 6 mounds; sprinkle with remaining powdered sugar.
Bake at 250º until crust forms but center is soft, about 25 minutes.
Cool, transfer to an airtight container, and place in the freezer until ready to use, up to 3 weeks.
Sauce: Combine the passion fruit juice and cubed mango with sugar (if needed) in a blender; add water if mixture is too thick. Place a pool of sauce onto each of 6 dessert plates. Place meringue on top of sauce. Drape mango slices on the plate, and add a small scoop of granita. Serve immediately.