I don't think there is a correlation between vegetarianism and atheism, but there definitely is one between some types of veganism and atheism.
Early followers of all the major religions were basically vegetarians, and they also drank cows milk (except for Buddhists - who are technically atheists anyway since they reject the authority of Vedic scripture.)
The Vedic culture of ancient India is arguably the original religion,and cows milk was highly valued as "liquid religiosity" for being the only food that can nourish the finest tissues of the brain needed for spiritual thinking. The cow was - and still is - considered a special type of mother for humans, and when cows are lovingly cared for and protected they are happily give their excess milk for human consumption. Cows milk - without harmful additives and processing - is very healthy in moderate amounts..
Vegans and Hare Krishnas share ethical concerns about abuse and unnecessary killing of cows by the dairy industry, but some vegans put out speculative propaganda that cows milk is unnatural and unhealthy for human consumption.
From my perspective as a Krishna devotee, I consider rejection of cows milk as a good food for humans to be more-or-less an atheistic principle - since Krishna specifies in Bhagavad-gita that cows milk - along with vegetarian food - is in the mode of goodness.