Filling: In saucepan, bring blueberries and sugar to boil over medium-high heat, stirring often. Reduce heat and simmer, stirring occasionally, until berries are tender, 5 to 7 minutes. In small bowl, whisk cornstarch with lemon juice and vanilla; whisk into blueberries. Boil, stirring, until thickened, about 1 minute. Scrape into bowl and refrigerate for 1 hour.