Pour the popcorn and nuts in a large paper grocery bag. Be sure to pick out any un-popped kernels. Place the brown sugar, butter, corn syrup, salt and vanilla in a large microwave safe bowl. Microwave on high for 2 minutes. Stir, then microwave another 2 minutes, until boiling.
Immediately stir in the baking soda and pour over the popcorn, in the bag. (The baking soda will cause the caramel to foam up.)
Fold the top of the paper bag over a couple times and shake well. Place the bag in the microwave and cook for 1 minute. Shake the bag vigorously again and microwave for 45-60 more seconds. Check at 45 seconds to make sure it's not burning.
Open the bag and pour the caramel corn out on waxed paper. Break up clumps if needed. Cool completely to harden. *Every microwave is a little different. After the caramel corn cools completely, if it does not crisp up, put it back in the bag and microwave for another 30-45 seconds.