Place the sugar, corn syrup and water in a saucepan and heat over medium heat, whisking constantly until the sugar dissolves. Attach a candy thermometer to the pot and boil until the mixture reaches 190 degrees F, about 2 to 3 minutes. Stir in 2/3 of the pistachios, the butter and the vanilla extract. Continue to stir even though the mixture may thicken – it will even out as it heats. Cook, stirring often, until the mixture is at 300 degrees F, about another 3 minutes. Sprinkle the baking soda into the mixture and stir well (it will bubble!) and turn off the heat. Immediately pour the mixture onto the greased parchment and spread it into a thin layer. Let cool completely.