Combine sugar and syrup in a Dutch oven; cook over low heat until mixture reaches soft ball stage (240°). Add peanuts; cook, stirring constantly, until mixture reaches hard crack stage (300°). Remove from heat; stir in soda, water, and vanilla.
Spread mixture onto a warm, buttered 15- x 10- x 1-inch jellyroll pan. Allow peanut brittle to cool; break into pieces.