Reacting to stimuli like the colour of light is irrelevant. My phone camera would fall into the same category, then. A light switch reacts to getting pressed and turns on a light, it's reacting to a stimulus.

What matters is sentience, which plants cannot possess, since they don't have a central nervous system. And even if they did, a diet that includes meat takes more plants, since those animals have to be fed plants in order to raise them.

They all make it up as they go along. It's very similar to religious beliefs in the way it is personal. Each has their own set beliefs on where to draw the line of what is vegan and what is not

The extent to which we are tied to every living thing on Earth means that many vegans have set impossible goals.

Regarding these two, is this any different from human rights? Where people draw the line regarding slave labour, child labour, which type of humans they care about (considering racism, homophobia, trans phobia, ableism etc). I'm sure lots of people have impossible goals regarding human rights, but working to get as close to those as possible is still sensible.