Line a 10x15 inch baking pan with parchment paper and set aside.
Place sugar, syrup, and water in a 2 quart with candy thermometer attached and bring to a boil over medium heat. Slowly stir in the butter a little at a time. Let mixture boil, stirring occasionally.
When mixture reaches 250 degrees (when you drop a little of it into cold water it forms pliable strands), add the peanuts. Stir constantly for 7-10 more minutes until mixture reaches 300 degrees (when dropped in water it forms brittle strands). Immediately remove from heat, add the vanilla and baking soda, and stir until evenly combined. Pour onto parchment lined baking sheet and spread evenly.
Let cool and set. Use a mallet or back of a spoon to crack the brittle into pieces.