In a large, heavy pot (not iron), combine the milk, sugar, and vanilla, and place over medium heat. Stir regularly until the milk comes to a simmer and sugar is dissolved. Remove the pot from the heat and add dissolved baking soda; it will bubble up at this point, especially with goat's milk. When the bubbles have subsided, return it to the heat.
Adjust heat so that the mixture is simmering briskly but not boiling. Cook, stirring regularly, until the mixture turns pale golden, about one hour.
You will now need to stir the milk more regularly as it begins to thicken and turns a caramel-brown color. Don't allow the milk to stick to the bottom of the pot. You can drop a few drops into a small glass of water. If a soft ball forms, the cajeta is ready.
If you take the pot off the heat and allow the cajeta to cool, it should be a medium-thick sauce. If it's too thick, add hot water, 1 tablespoon at a time until it is the proper consistency. If it is too thin, return to the heat until it thickens.
When the cajeta is cool, remove the vanilla bean. Strain the cajeta through a fine mesh strainer into a bowl or wide-mouthed jar, then scrape the seeds from the vanilla bean into the cajeta. Refrigerate until ready to use. Cajeta is best served warm.