You can also use apple sauce. You really just need moisture without being pure water. It will be a bit more likely to stick, and it won’t have exactly the same texture, but it will be better for you. I say this because half a cup of butter is a lot of butter. It will likely change the texture as well, and good lord your arteries! Then again, butter makes everything better, so it may make a far better cake than using the vegetable oil would. Tell us how it turns out.