Combine the first four ingredients in in a large saucepan and place over medium heat. Bring to a simmer, stirring occasionally, until the sugar has dissolved.
Once the sugar has dissolved, add the baking soda and stir to combine. Reduce the heat to low and cook uncovered at a bare simmer. Stir occasionally. Continue to cook for 1 hour.
Remove the vanilla bean after 1 hour and continue to cook until the mixture is a dark caramel color and has reduced to about 1 cup, approximately 1 1/2 to 2 hours.
