Re:
Surely there's scope for different possibilities here: some people probably do claim to be vegan because it's fashionable; some try to be vegan for health reasons; others try to be vegan for ethical reasons, and did so long before it became a celebrity fashion.
On the other hand, it probably is impossible to be truly vegan, especially in the modern world. But there is more than one response to that: either you can accept it and just try to live a relatively harmless life, but knowing that some animals will be harmed in the process; or you could take veganism as a 'regulative ideal' meaning that you try to be vegan, knowing that you will not completely succeed, but believing that you will cause least harm if the ideal of veganism regulates your actions. In other words, you set a goal you can never attain but you pursue it anyway because you believe it will produce better results than not trying to pursue it at all. Each one is a respectable approach.
The issue of whether plants should receive moral consideration is an interesting one and the point about carrots is not unreasonable. There are various aspects to the issue. On the one hand, it seems to make it easier to explain our consciousness if we can say that everything has some degree of consciousness, even if it is fairly minimal; otherwise, we have to explain how completely unaware matter can produce awareness in us, and that's not easy at all. On the other hand, even if plants are not conscious, we might say they deserve moral consideration because they are alive, and living things are fundamentally different from non-living things for reasons that would take too long to explain (telos). However, the problem with trying to argue for the moral consideration of plants is that their well-being is so easily, routinely and probably rightly overridden by other concerns, that the idea can appear daft. However, carrying little weight in our moral judgements is not the same as carrying no weight.
The point is, as I think LGF was suggesting, whether you try to be vegan, vegetarian or omnivorous, it's moral shades of grey, not black and white.
Lecture finished.