Preheat oven to 250. Put popcorn in a large, 4 inch deep buttered baking pan. Keep warm in the oven. Combine sugar, cornsyrup, water, butter and salt in a large saucepan. Stir over medium heat until sugar is dissolved. Cook until mixture reaches 250 on a candy thermometer, stirring frequently. Remove from heat. Quickly stir in vanilla. Remove popcorn from oven. Pour syrup mixture over popcorn, stirring to mix well. Form into balls. Makes approximately 8 balls.