You can certainly substitute with vanilla extract, but you won’t get the same intense vanilla flavor or the flecks of vanilla beans throughout. The Oat Fiber is really important in this recipe. I use it in so many recipes, because of the amazing texture it gives whatever I am baking! It is hands down, the best ingredient I’ve baked with and is really worth buying. I use Lifesource Foods Oat Fiber 500 and I buy it in the 16 ounce bag on Amazon (link in recipe). The bag lasts a while because you don’t need to use much in your recipes. Oat Fiber, is not the same as Oat Flour, which is made from grinding oats. Oat Fiber is made from grinding the outside husks of oats and is pure insoluble fiber. Insoluble fibers do not dissolve in water and are not broken down in the digestive tract.