Disclaimer: I'm not a vegan.
they gladly eat food grown by slaves
They don't. Certainly not more then carnivores. In fact much more vegans buy local, organic and/or fairtrade food then carnivores.
But even if: buying slave products is not what veganism is about. Its completely useless to judge the specific decision to be a vegan by any other lifestyle aspects the particular individuals have, as long as its not tied to this very decision. If Hitler was the only vegan on earth, that wouldn't mean veganism is a bad thing.
Whow. I totally missed that I can export to CMYK.