For veganism the model would be spreading, like a disease, rather than reproducing as a trait of an organism. In such cases natural selection pressure is exerted against the disease organism indirectly through it's effect on the host's reproductive success.
The Jain sect in India are the closest thing to vegans who have a long track record and they aren't strictly vegan (they generally will eat dairy if it's been harvested in an approved way). Their numbers have been more or less stable over time. There are about five million worldwide today.