Veganism is a boycott of animal products, not the businesses that supply them. If vegans were boycotting every business that supplies animal products, we wouldn't be able to go to grocery stores. That would be absurd.
Nerak is right. KFC getting a vegan option is fantastic for veganism. It's putting veganism in the mainstream and making it more accessible and convenient for the average person. By buying vegan food from non-vegan restaurants, we're telling them that veganism is profitable- making it more likely to spread further and become cheaper and more widespread. If we didn't support it, they'd probably take it off the menu, which would be worse. It's all a game of economics.
Plus, it's unlikely that they'll recycle all of the money they gained from the vegan burger into producing meat.