I'm assuming that the LessWrongers interested in 'should I be a vegan?' are at least somewhat inclined toward effective altruism, uilitarianism, compassion, or what-have-you. I'm not claiming a purely selfish agent should be a vegan. I'm also not saying that the case is purely intellectual (in the sense of having nothing to do with our preferences or emotions); I'm just saying that the intellectual component is correctly reasoned. You can evaluate it as a hypothetical imperative without asking whether the antecedent holds.
the LessWrongers interested in 'should I be a vegan?'
I am sorry, where is this coming from?
I'm just saying that the intellectual component is correctly reasoned
At this level of argument there isn't much intellectual component to speak of. If your value system already says "hurting creatures X is bad", the jump to "don't eat creatures X" doesn't require great intellectual acumen. It's just a direct, first-order consequence.
I'm currently unconvinced either way on this matter. However, enough arguments have been raised that I think this is worth the time of every reader to think a good deal about.
http://nothingismere.com/2014/11/12/inhuman-altruism-inferential-gap-or-motivational-gap/