Boil sugar, Karo syrup, and water until thread spins (250 degrees), Add Peanuts; stir constantly until temperature reaches 290 degrees. Remove from heat. Add butter, vanilla, and soda. Pour onto buttered cookie sheets, spreading as thin as possible with spoon.
If thinner brittle desired:
After two minutes, move to wax paper and spread until desired thickness.
Add water to pot and return to heat. Sugar will become soft and make clean-up much easier.