Skeptics are hardly ever vegan. Many skeptics hate vegan atheists.
If atheists are simply reacting to the restrictions and dogma of religion (especially the Abrahamic religions) then these atheists are … blank. That being said, the last thing they want to do is give up the freedom that they have realized. They have become what I call selfish atheists. They care little about people, other species or the world. They are basically anti-theists. These atheists hate anyone that, by example, threatens their freedom. They make fun of vegans and use their fine tuned anti-religious argument techniques on vegans to "save" these vegans. This is strange since veganism is not a religion.
If on the other hand, atheists are nonbelievers because it is rational to think that way then they are open to other rational ideas as well and they find it absolutely necessary to form ethics about their behavior in the community and ethics toward other species and the world. Vegetarianism and veganism give these atheists pause. They do not dismiss vegans and vegetarians, rather, they embrace all atheists of integrity that strive to live by non-religious ethics. These people are fully functioning atheist. They may even become vegan atheists.
In the USA there has been a long history of religious abuse and therefore there is a large number of rebellious atheist also known as atheist warriors or simply zealous skeptics. In a war these atheist champions are good to have around, however, when peace breaks out community minded ethical atheists are needed.
Here in the Inland Empire (Riverside, CA) I would say there may be 1 in 50 vegan/vegetarians in our atheist group of 700 people.