Heat oven to 300°F. Line a large rimmed baking sheet with parchment.
Spread pretzels evenly on the prepared pan and set aside.
Melt butter in a medium saucepan over medium heat. Stir in brown sugar.
Bring to a gentle bubble, stirring, and cook 3–4 minutes until glossy and smooth.
Remove from heat. Quickly stir in vanilla and baking soda until foamy.
Pour the toffee over pretzels. Toss with a spatula to coat as evenly as possible.
Bake 12 minutes, stirring once halfway to redistribute the coating.
Scatter toffee bits over the pan and bake 2–3 minutes more to set.
Cool completely on the pan. Break apart and finish with a light sprinkle of sea salt.