Grease large rimmed baking sheet; mix warm popcorn with pecans on sheet. Set aside.
In saucepan, whisk together sugar, butter and corn syrup over medium-low heat until sugar dissolves. Increase heat to high; boil, without stirring, until candy thermometer registers hard-ball stage of 255°F (124°C), or 1/2 tsp (2 mL) syrup dropped into very cold water forms rigid ball that is still a little pliable, about 4 minutes. Remove pan from heat. Stir in vanilla, salt and baking soda.
Gradually pour over popcorn mixture, stirring gently until combined. Let cool completely. (Make-ahead: Store in airtight container for up to 1 week.).