Cook first 3 ingredients in a large heavy saucepan over low heat, stirring until sugar dissolves. Cover and cook over medium heat 2 to 3 minutes to wash down sugar from sides of pan. Uncover and cook, without stirring, until candy thermometer registers 238° (soft ball stage).
Remove from heat, and add butter, vanilla, and salt (do not stir). Cool to 175°. Stir in pecans, and beat with a wooden spoon until mixture thickens and begins to lose its gloss. Spread into a buttered 8-inch square pan. Cool until firm. Cut into squares.