Your heat was probably too high, or you cooked it a little too long. Keep the heat on medium and let it bubble, the change you will notice in the color is subtle, takes a while (be patient!) and when it starts to happen, it happens fast. Stir frequently (yet gently) to avoid burnt bits, and make sure to get caramel out of your pot after the vanilla extract is whisked in, or it will continue to cook and may burn.