The economics of veggie proselytism actually look pretty good.
Animal Charity Evaluators found that for every $11 spent in online ads, you could make one person go to a website, sign up with their email, and pledge to go vegetarian for a year. The average vegetarian [1] produces 1.5 metric tons less CO2eq per year, so this means that the online ads decrease carbon at a cost of $7.40 per ton. That's cheaper than buying typical carbon offset certificates ($10-$15 per ton).
Maybe it's too optimistic to assume that everyone who makes a pledge on the website actually stays vegetarian for a year, or that they were average-level meat consumers before. Still, animal farming is bad enough that reducing it is probably cost-competitive with other current CO2 reduction approaches.
[1] I'm not sure if their use of "vegetarian" means "completely vegan".