I believe you are missing the forest for the trees. It is bringing up the question of what defines self will. It is unrelated to veganism in all but text.

An easy example is dogs. We have bred dogs for centuries to love doing work for us. If they hated doing the work, it would be easy to call it cruel. If they loved it by nature, it would be easy to call it kind. But since we created them into a thing that loves the work we need them for, where do the ethics fall?

Should we prevent them from doing what brings them joy? Should we make use of this win-win situation? If it is the latter, we are quickly approaching the ability to morph every species into something that gets joy from doing our work.

Dogs we changed by accident. The next one will not be an accident. Is it still a beings free will if the game was rigged from the start?