In a large, heavy-bottomed saucepan, combine the granulated sugar, corn syrup, water, and salt, stirring very gently while trying to avoid getting any sugar crystals on the sides of the pan. Cover the pot and bring to a boil over medium-high heat, until the sugar has melted and the mixture is clear, 3 to 5 minutes. Uncover, and then cook until the sugar has turned a pale golden color, 4 to 5 minutes more, and registers about 300F [150C] on a candy thermometer. Turn the heat down slightly, and cook for a few minutes more until the sugar is golden and registers 350F [180C]. Remove the pot immediately from the heat and add the vanilla, stirring to combine.