The most accepted definition of veganism goes:

"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."

Emphasis added. Your argument is valid, in that modern medicine, vaccines, and animal testing are all challenges vegans need to address. This is something that's a lot harder and less clear cut than diet or not wearing certain clothing. Not every vegan agrees on what the best course of action is either, but most lean toward at least not being anti-vaccine. Self-preservation pretty clearly counts under the possible and practical part of the definition.

But that does not invalidate the very real differences and good that does come from going vegan, for ourselves, for the animals who are spared a life of hell, and for the planet.