Carnivorous plants grow in places with soil that doesn't offer much food value. Carnivorous plants grow mostly in wet areas, from sea level to the mountains. North America has more carnivorous plant genera than any other continent.
Example:
Some pitcher plants (Nepethenes) trap frogs, and when the frogs are digested, all that remains are little matching frog slippers since the skin on frogs' feet resists digestion.
Plants like the Venus fly trap catch flies and other insects to obtain the proteins that they need. By definition a vegan should not be eating anything meat-based. That is why they do not use milk even though one does not have to kill a cow to obtain its milk.