Wash the blueberries and pat dry with paper towel. Wash and hull the strawberries, pat dry. Slice off the bottom of each strawberry to make a flat end. This allows the strawberries to stand upright on a platter. Mix together the marscarpone cheese, lemon zest, lemon juice, confectioners' sugar and vanilla. Spoon the mixture into a resealable bag and snip off one of the corners of the bag (you can also use a pastry bag). Pipe the marscarpone filling into the cavity of each strawberry and top with a blueberry.