I’ve made soft caramel candies and caramel sauce. Nothing fancy, just vanilla flavored. They usually call for cooking the sugar with a small amount of water until it turns a deep amber color. Then you add in more liquid—water, milk, cream, butter… it bubbles up like crazy.
For the sauce, the recipe called for just pulling the mixture off the stove at that point and adding flavorings, like vanilla extract. I’d do that with the liqueur, too. If you’re using quite a bit of it and don’t want to thin the sauce down too much, just reduce the amount of other liquid you add.
For the soft caramel, the recipe called for further cooking to concentrate the mixture. For that one, you needed a candy thermometer. Again, my recipe called for adding vanilla extract after the caramel was taken off the heat. If you’re adding a whole lot of flavoring liquid as you take it off the heat, you might want to cook it to a slightly higher temperature to make up for diluting it more with the flavoring.