Mix together the peanut butter, syrup, and brown sugar. Microwave for 2-3 minutes on high until melted together. Stir in the ground flaxseed and vanilla, then pour the mixture over 4 cups of puffed cereal. Mix well and add more cereal as needed. Coat a 9x13 inch baking pan with cooking spray, then spread the mixture into the pan and press down to form a dense sheet. Let the mixture sit for about 5 to 10 minutes then cut into bars.