I prayed to GuanYin and obtained a response. I therefore have been eating vegetarian in order to repay her kindness. Unfortunately, I inadvertently ate a non-vegetarian cake. What should I do?
You can eat vegetarian another day to make up for it. When it is unintentional then it is only a slight offense. Now that you have repented, it has been erased.
A word of caution: do not try so hard because it might cause others to think that we Buddhists are extreme. Please consider yourself as still trying to learn to eat vegetarian.