Cook sugar, syrup and water until mixture thickens and bubbles (about 230 degrees). Add butter and pecans. Stir continuously and continue cooking until nuts begin to brown or start popping (290 degrees). Remove from heat. Add vanilla and soda and stir thoroughly. Pour quickly in several pieces on large buttered surface. Do not spread with knife or spoon. Allow to cool slightly. Stretch and pull with hands until very thin. (The thinner it is stretched, the more brittle it will be.) When cooled and hardened, break into small pieces. Store in airtight container.