The definition from the vegan society is:

Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals.

Is climate change cruel to animals? It’s not intentional harm, but it causes suffering. So it depends on the ethical framework (deontology - utilitarianism spectrum).

Going on vacation by plane arguably isn’t vegan from a utilitarian perspective. Deontologists might still see it as vegan.

If someone needs to drive a car and can’t afford an EV, it’s not practical to avoid fossil fuels in this case. So that would be vegan either way.

I think the “avoiding as far as possible and practicable” principle also makes a lot of sense for the use of fossil fuels by environmentalists.