There are a few reasons why your vegan vanilla cupcake frosting can fail. Let me explain the reasons below, or jump to the next section for my troubleshooting tips. 1. Over-beating the ingredients - you beat the frosting too fast for too long. 2. Warm environment or ingredients - you must use vegan butter straight from the fridge. 3. Too much soy milk - adding too much soy milk or any dairy milk curdle frosting. 4. Old batch of powdered sugar - poorly stored powdered sugar forms lumps and crystals that you can feel in your buttercream. 5. Too sweet - you added too much powdered sugar to balance a runny frosting.