Combine the remaining strawberries and sugar in a 2-quart saucepan over medium-high heat. Bring the mixture to a boil and cook, stirring occasionally, for 5 to 10 minutes, or until the sauce thickens. Remove from the heat and stir in the reserved strawberries and vanilla extract. Allow the sauce to cool to room temperature before using. Leftovers can be stored in an airtight container or jar in the refrigerator for 2 to 3 weeks.