definitionally you aren’t vegan if environmental impact is your terminal goal. It would be like saying you’re Christian for the cathedrals or something.

Veganism is a philosophy and life practice of trying to minimise harm to other earthlings. It can involve environmentalism as an instrumental goal, that is protecting the environment to avoid mass suffering, but a world of perfect environmental preservation where all ants have depression would be unacceptable to a vegan but not to an environmentalist.

Many people with environmental goals adopt a plant based diet and/or lifestyle.