Being an atheist and an animal activist are not mutually exclusive, as a matter of fact an atheist is a person who acts without delusions, and takes responsibility for his moral choices. This should inevitably lead the atheist to veganism and animal activism.
What I say is this : if you define a good person as someone who does not cause slavery, exploitation, torture or unnecessary suffering then if you consider yourself a good person, in essence you are all vegan. If you are not a hypocrite then you become vegan. A lie that is preferable to the truth is still a lie.