Preheat an oven to 350 degrees F (175 degrees C).

Mix the butter, crushed saltines, and walnuts together in a bowl, and press the mixture into the bottom of an 8x12 inch baking dish.

With an electric mixer, beat the egg whites, sugar, cream of tartar, and vinegar in a large metal or glass bowl until the mixture forms stiff peaks, then beat in 1 teaspoon of vanilla extract. Spread the meringue mixture over the prepared crust, and bake in the preheated oven until the meringue is golden brown and firm, 10 to 15 minutes. Let the crust cool.

Mix together the blueberry pie filling and lemon juice in a bowl, and spread the mixture over the cooled meringue.

Beat the cream and 1/2 teaspoon of vanilla extract together with an electric mixer, gradually adding confectioners' sugar, until the whipped cream forms fluffy peaks. Cover the blueberry pie filling with the whipped cream, and refrigerate until served.