Line 2 large sheet trays with waxed paper; set aside. Melt butter in a medium pot over medium heat. Stir in brown sugar, agave, curry powder, and salt; bring to a rolling boil, stirring constantly. Continue to boil, without stirring, 4 minutes; remove pot from heat and carefully stir in vanilla and baking soda until well combined.
Place popcorn in a large bowl. Working quickly, pour hot caramel over popcorn and use a spoon to coat evenly. Transfer to reserved sheet trays and spread out in an even layer. Set aside to cool completely, then serve.