Yes, both atheists and vegans are self-righteous nuisances who tend to cause problems at family reunions. Other than that striking similarity, this question seems a strange one; after all, what could atheism and animal rights possibly have to do with one another? Veganism is very much a belief in something, while atheism is a lack of belief in something.
Despite these differences, two connections come to mind. First, the two most common defenses (though not the best) of eating meat are closely linked with a God-created universe. The first of these defenses is that nonhuman animals were put here by a God to be used by humans and that they do not have souls so it is alright to use them. The second defense claims that humans have some special characteristic, such as intelligence or language, that all other animals do not. This claim, that one species possesses some characteristic wholly absent in ALL other species flies in the face of any theory of evolution, which maintains that differences between species are quantitative, not qualitative. So if nothing else, atheists should perhaps be more receptive to arguments for veganism.
Knowing many vegans and many atheists, the other thing I’ve noticed in common between them is not some argument they’ve heard or philosophy they have, but the way in which they came to their respective beliefs. Most atheists I know can relate some story about having been raised religious and then being confronted with arguments for atheism. We had a belief mostly because our parents had the same belief and because of convention and the culture we grew up in. Questioning these beliefs and coming to an alternate truth was likely a long, uncomfortable process that didn’t please your relatives. Many vegans I know share similar stories of confrontation, discomfort, and parental and social disapproval. So perhaps another connection between atheism and veganism is a common process by which one becomes atheist or vegan.
The link may merely be that both atheists and vegans are likely to be skeptical of the beliefs and traditions with which they grew up and that this skepticism leads to an eventual rejection of the cultural norm, whether it is theism or carnivorousness.
Well, that and we’re both pretentious.