If we're including microscopic animal life, then we can never truly be vegan

Other way around comrade. You could theoretically murder a cow or, like, a whole ass human and eat them and still be vegan. If that was the only way for you to survive in the circumstance you were in, it's vegan. The point is that in the modern world we don't have to do stuff like that. If you're avoiding harm to animals as much as you can, then you're vegan.