Preheat the oven to 200°F. Line a large baking sheet with parchment paper and spread the pretzels evenly over it.
Add the butter, brown sugar, and light corn syrup to a large saucepan over medium heat.
Bring the mixture to a boil and stir continuously for 5 minutes.
Remove from the heat and carefully stir in the vanilla extract.
Add the baking soda and stir until combined. The mixture will bubble up, so use caution.
Quickly pour the hot toffee mixture over the pretzels and sprinkle half of the toffee bits over them.
Bake for 1 hour, stirring the pretzels every 15 minutes.
Sprinkle the remaining toffee bits over the pretzels and bake for another 5 minutes. Remove from the oven and cool completely.
Cool completely, then break into pieces and serve.