The only instance of using "just Vit E" would be okay would be if there is only oils/butters in the product, any water at all, it will not be sufficient. Keep in mind, Vit E alone will not keep the product stable if water is introduced to the product after you make it, folks dipping their fingers in the product with wet hands or a shower running behind them, etc. if you are making whipped butter for yourself, I wouldn't add a preservative, vit E should be sufficient. For customers, better be safe and use what you need and have it tested.