Add vinegar to half n half and let it sit for about 5 minutes. (The alternative is to use 3/4 cup of buttermilk.)
Put milk/vinegar mixture, brown sugar, granulated sugar, cinnamon, nutmeg, butter, corn syrup and baking soda in a large pot. The mixture foams up so make sure your pot is large enough.
Bring to a boil over medium heat and then turn down to low. Cook on low for about 8 minutes, stirring constantly.
I used a flat bottom wooden spoon to stir and scrape the bottom. As the caramel cooks, it turns into a lovely golden color. It will darken more when the vanilla is added.
Remove from heat and add the vanilla.
Mix well. The syrup will thicken as it cools.