1. Line 1 large, rimmed baking sheet with nonstick aluminum foil or parchment paper; set aside. Combine popcorn and almonds in a very large bowl; set aside.
2. Combine butter, corn syrup, and brown sugar in a large (3-quart), heavy pan. Cook over medium-low heat, stirring often, until butter melts and sugar dissolves. Increase heat to medium, and bring to a boil. Cook, without stirring, until mixture reaches 254° (very hard ball) on a candy thermometer. Remove from heat.
3. Stir in salt, baking soda, and vanilla extract. (Mixture will bubble.) Quickly pour mixture over popcorn and nuts; toss gently to coat. Transfer popcorn mixture to prepared baking sheet.
4. Bake at 250° for 45 minutes, stirring occasionally. Remove from oven; cool completely. Store in an airtight container for up to 2 weeks.