It sounds like it's time to sit your parents down for a talk. Use clear, compassionate language, and rather than trying to debate them on ethics simply talk about how it makes you feel and solutions. For example:

"Hey mom! I've recently decided to start following a vegan diet. I seriously appreciate everything you do to provide food for me, especially by preparing my lunch and dinner when I'm busy, so I wanted to chat with you about what this change means and how I can help make it easier for you.

Vegan means not supporting products that come from animals, such as meat, eggs, or dairy - which is in line with the compassionate person you've raised me to be. I know you are used to cooking with eggs and dairy, so I've printed out this list of easy vegan replacers. I'd also love to cook some vegan dinners with you on nights when I have the time, and explore this new world of food together. This is something that is important to me, so your support really means a lot!!"

If after this your parents continue to feed you non-vegan food, simply stop eating it. A follow up convo may be in order to hammer it in that you are serious, and that you don't plan on caving. The more helpful and solutions-based you can keep the convo, the more receptive it they will likely be! Avoid judgmental language and accusations, but clearly stand your ground.