Question: As I wish to commit spiritually to the vegan diet, is it okay to eat vegan food that is cooked or mixed with non-vegan food?
Answer: Buddhists do not see spiritual defilements to be incurred merely by taste; but by whether there is consumption with greed, hatred and delusion. Thus, this is not a big issue to eat vegan food that is with non-vegan food, as long as there are no actual animal contents or their taste. If animal contents are in the food, it becomes non-vegan straightaway. If such food is demanded for, it thus becomes as good as actively demanding non-vegan food to some extent, which is what you do not wish to do in the first place.
If there is tainted taste of non-vegan food, it becomes non-vegan in taste, which might give rise to greed for the actual non-vegan food. This too is what you wish to avoid as you want to nurture more compassion for animals instead of greed for their products, which do result from them being exploited and eventually killed. To avoid these two potential problems, it is best to go for directly vegan food. (Some practise washing of tainted food with hot water to prevent wastage.)
Note that beyond mere veganism, Chinese Mahayana Buddhists are encouraged by the Buddha in the Surangama Sutra and other teachings, to consume the Maha-vegan (大乘净素) way – without meat, other animal co and by-products, five pungent roots (garlic, leek, onion [spring onion, shallot, green onion or scallion], chives, asafoetida) and alcohol. ‘Maha-vegan(ism)’ is a term to define these considerations together, expressing ‘great(er)-veganism’. (The roots spur greed and hatred while alcohol spurs loss of mindfulness.)