In a saucepan combine salt, butter, brown sugar. Continue stirring until it comes to a full boil and whisk continually for 5-10 minutes, until mixture hits soft-ball stage (put a few drops in a glass of icy water until the drops form a soft ball).
Remove mixture from heat and mix in vanilla extract and baking soda. Continue stirring as the mixture goes puffy.
Drizzle caramel over popcorn and lightly stir until evenly coated.