Use a heavy pan (iron skillet is best). STIR CONSTANTLY! Boil sugar, water and syrup until it threads (soft crack on thermometer 286 degrees). Add the peanuts. Stir constantly. Boil until the color of butterscotch (don't let it get too brown or it will taste burnt). Add the butter/oleo and stir thoroughly. Remove from heat. Add baking soda moistened with the vanilla. Stir thoroughly. Pour onto buttered pan (use REAL butter on the pan). Spread it out while it is hot. Using scissors pull away from center and cut into pieces immediately. Keep in covered can or freeze in Ziploc bag.
(Photo: peanut brittle could be darker, this was my first batch)