Generously butter 2 cookie sheets. In large saucepan, combine sugar, corn syrup and water. Cook until sugar dissolves, stirring constantly. Cook until candy thermometer reaches hard-crack stage (300°F.), stirring occasionally.
Stir in margarine and mixed nuts. Remove from heat. Stir in vanilla and soda. Pour onto buttered cookie sheets; spread as thin as possible. Cool 30 minutes or until competely cooled. Break brittle into pieces. Store in airtight container in cool dry place.