I just found out about this environmental campaign called We Add Up ( http://www.weaddup.com) and I was excited to see that veganism is an aspect of it. Here is what they say on the website:
WE ADD UP is a global campaign using organic cotton t-shirts that literally “counts you in” to help solve the climate crisis. Every shirt is printed by hand with a unique number. YOUR number is your position in our sequential global count of people who are taking steps to do their part. As the count grows, we demonstrate to the world that “WE ADD UP.” On the back of each shirt is a word or phrase that describes an action almost anyone can take to reduce their carbon footprint - the contribution their lifestyle makes to greenhouse gases - such as, Unplug, Lights Off, Carpool, Hybrid, Bike, Buy Local, and 27 others. You choose which action you are committed to doing and get counted in. No one can do everything. Everyone can do something. And, WE ADD UP.
Their vegan shirt:
http://www.weaddup.com/images/P/SS-026.gif

Anyway, I don't know much about the project but it seems cool. And they say "Three dollars from the sale of this tee will be donated to a non-profit organization working to implement the solution this shirt promotes. Please email us to recommend a worthy organization."  It's probably better to just give the $25 directly to the organization you want, but wearing vegan shirts is always a good thing too, since it can get people talking.