Cook until mixture just barely begins to boil. Remove from heat and stir in the peanut butter and vanilla until smooth.
Pour over popcorn and stir until popcorn is evenly coated.
You will need about 20 to 24 cups of popped popcorn, depending on how gooey you want yours. You don't have to use microwave popcorn, I just do because it's convenient. I have used my hot air popcorn popper before and popped my own popcorn. 1/4 cup of popcorn kernals will yield about 8 cups of popcorn.
This recipe can easily be halved if you don't want a lot of popcorn.