Preheat oven to 250°f. Line large rimmed baking sheet (also known as a jelly roll pan) with parchment paper.
Place popcorn, almonds and pecans in large heatproof bowl.
In medium saucepan, bring brown sugar, butter and corn syrup to a rolling boil over high heat. Reduce heat to medium-high and bring mixture to soft-crack stage (270°f to 290°f on an instant-read thermometer) without stirring.
Remove from heat and stir in baking soda and vanilla.
Pour caramel mixture over popcorn and nuts and stir until completely coated.
Scoop out onto baking sheet, arranging in an even layer.
Bake for 1 hour, stirring every 15 minutes. Let cool completely, then break apart into desired size of clusters.
Store in an airtight container at room temperature for up to 5 days or in the freezer for up to 3 months.