In a bowl, dissolve gelatin in water. Stir in strawberries, pineapple, bananas and nuts. Pour half the mixture into a 13-in x 9-in x 2-in dish. Refrigerate for 1 hour or until set. Set remaining gelatin mixture aside.
Combine the sour cream. sugar and vanilla extract; mix well. Spread over the chilled gelatin. Chill for 1/2 hour.
Spoon remaining gelatin mixture over the top. Chill overnight.