1. Begin by heating the syrup in a small pot over medium heat, then add the sugar to the pot. Stir continuously for a few minutes; you might start to notice it bubbling at the bottom. If it starts to become frothy, turn down the temperature some.
2. When it starts to look like it's boiling, add the margarine. Stir until evenly mixed, then add the vanilla, still stirring. If you notice that the syrup/sugar mixture does not boil, you can turn up the heat.
3. Meanwhile, in a very large bowl, measure out about 5 cups cereal. Pour the heated mixture over the cereal. Stir all ingredients with a heat resistant spatula making sure to evenly combine.
4. You might want to add the remaining cup or more of the cereal, depending on the amount of saturation you're looking for (I find about 6 cups cereal allows all the cereal to be covered in syrup but not too gooey; still crispy.) Lay out a 9x13" piece of wax paper, and lightly spray with oil.
5. When mixture is stirred to satisfaction, pour the cereal on to the wax paper, using the spatula to push it into a large rectangle, approximately 1" or so thick. Allow the rice crispies to cool until hard, about an hour or so (although sampling of a chunk of warm rice crispies is possible for instant gratification!)