Red Herring
Logical fallacyIntroducing a point that is not relevant to the question under discussion, drawing attention away from it rather than answering it.
An everyday example
“You asked why the deadline slipped, but have you noticed how hard everyone has been working?”
The same move in this debate
“Why worry about animals when there are people sleeping rough?”
Where this appears
Shifts from the choices actually in front of the person to an emergency that neither party is in, and which both would resolve the same way.
The constraints facing people without food access do not bear on the choices available to someone who has access. Two different situations are being treated as one.
Food miles are a small component of food emissions. Focusing the comparison on them diverts attention from production, which is where almost all of the difference lies.
Moves from diet, which is the subject, to medical research, which is a different question with a different balance of interests and which veganism's standard definition already handles separately.
Meta-ethical scepticism, if accepted, undercuts all moral claims equally and so provides no reason to favour one course of action over another.
The constraints facing people with limited access are offered as a reason bearing on the choices of someone who has full access. They do not bear on them.
A question about wild animal population dynamics is raised in answer to one about farming practices. The two are connected only at several removes.
Difficulty bears on how demanding a course of action is, not on whether it is justified. The objection answers a question of cost with the vocabulary of a question about rightness.
That someone stopped tells you about their circumstances, not about whether the ethical or health case holds. The two questions are being treated as one.