It's not a good idea to use regular non-dairy milk in a recipe that calls for vegan buttermilk. Vegan buttermilk is acidic so it reacts with the leavening agent (usually baking soda) in the recipe. So if you use regular non-dairy milk in a recipe calling for vegan buttermilk then your baked goods might not rise, or they may be dense. It could also make your baked goods not as moist or rich as they were intended to be.