Line a large (17-by-12-inch), rimmed baking pan with foil and spray lightly with cooking spray; set aside. Spray a large bowl (not plastic) with cooking spray and place popcorn and nuts in it.
In a medium saucepan, combine sugar, butter and corn syrup. Bring to a boil over medium heat, stirring constantly. Clip on a candy thermometer and boil, stirring occasionally, until temperature reaches 290 F (about 15 minutes). Remove candy thermometer and stir in vanilla.
Pour mixture over popcorn and stir to coat well.
Spread popcorn mixture in an even layer into prepared baking pan. Sprinkle with sea salt. Let cool completely before breaking into pieces to serve. Store in an airtight container.