Heat the butter, sugar, vanilla, salt, and baking soda in the slow cooker on high until melted, about 20 minutes.
Gently stir in the popcorn then cover and cook on high for 1 hour, stirring every 15-20 minutes. Remove lid and cook an additional 30 minutes, stirring after 15 minutes.
Transfer to a parchment lined baking sheet and spread to let cool, about 15 minutes.