In saucepan, bring partridgeberries, sugar and 1/4 cup (50 mL) water to boil over medium-high heat, stirring occasionally; reduce heat and simmer for 5 minutes. Add blueberries.
In small bowl, whisk cornstarch with 2 tbsp (25 mL) cold water; whisk into berries and simmer until thickened, about 2 minutes. Stir in vanilla. (Make-ahead: Let cool and refrigerate in airtight container for up to 4 days; reheat before serving.).