To prepare crust, combine crumbs, margarine, and sugar in a mixing bowl. Press mixture firmly in a 9" pie pan. Chill one hour, or until firm. Or, bake in a 350 oven for ten minutes or until lightly browned. Cool on rack before filling. In a saucepan, heat marshmallows and milk over low heat until marshmallows are melted. Cool ten minutes. In a mixing bowl, combine sour cream, cream cheese, and vanilla extract. Mix until smooth. Stir marshmallow mixture and fruit cocktail. mixtures together. Turn into crust. Chill until set.