I build robots as a hobby. My robots are aware of their their environment and their place in it. In that sense, they're "self-aware" but nobody would argue they're conscious.
The underlying problem is that we have all these words that imply a precision that doesn't exist. All we can say for certain is that consciousness and self-awareness are effects we experience. Beyond that, all bets are off. We can't even really say what these things actually are.
I would extend your "some vegans" to "most everybody" - who seriously thinks complex animals are unconscious automatons? And how would that even work? Tiny few-celled organisms surely aren't and where exactly the line goes we don't know, and like you say it depends on how the word is defined and used, but isn't "stimuli causes not just reaction, but qualia" pretty standard?
You'll speak with a Christian and he'll say that even a very early fetus is conscious. Speak with other people, and it isn't. Some vegans say animals are conscious. Others don't. It's possible there's something real behind all of those people's definition of consciousness, and the real problem isn't that some are right and some are wrong.
They just disagree because it means different things for them, they just happen to be using the same word. The meaning of "consciousness" is more of a reflection of the values of the speaker than something real.