In a bowl, dissolve gelatin in water. Stir in strawberries, pineapple, bananas and nuts. Pour half of the mixture into a 9x13 dish. Refrigerate for 1 hour or till set. Set the remaining gelatin mixture aside. Combine the sour cream, sugar and vanilla; mix well. Spread over chilled gelatin. Spoon remaining gelatin mixture over top. Chill overnight.