Combine marshmallows and cranberry sauce in a medium saucepan. Cook on medium-low heat, stirring constantly, just until marshmallows are melted. Remove from heat; cool 10 minutes. Refrigerate, stirring occasionally, about 1 hour or until mixture mounds slightly when dropped from a spoon. Beat whipping cream and vanilla in a small mixing bowl until stiff peaks form. Stir cranberry mixture until blended; fold gently into whipped cream. Pour into crust. Chill until set, at least 5 hours. For a cool treat on a hot day, try freezing the Cranberry Mallow Pie until firm. To serve, garnish slices with additional sweetened whipped cream, if desired. Makes 6 servings.