Combine pineapple juice and cornstarch in a heavy medium saucepan; whisk until smooth. Cook, whisking, over medium-high heat until the mixture thickens, about 2 minutes. Stir in pineapple, orange juice, brown sugar and butter. Bring to a boil and cook, stirring gently, for 1 minute more. Remove from heat. Stir in vanilla. Let cool for 15 minutes. Serve warm.