I'm not vegan or vegeterian. But I feel like that "unless you are doing both" is kinda a wrong argument to make, even almost bad faith.
Because i could say "ah so you are anti work abuse? Curious, you are using a phone made by leveraging abuse"
I think a more honest argument would be to weight the possibilities and the outcomes.
Producing meat does increase the carbon footprint, and the same can be said for flying.
The idea is, how much can beign vegan reduce this footprint, and how much removing unnecessary flights can save? So overall, how much i am saving doing one of those, instead of both?
Also, eating is an everyday activity, flying usually isn't (in the example you reported)
I have no answers, i just didn't like the argument
Yeah we can agree on that one, of course cognitive dissonance is a thing so it's ok to point it out. (I love animals but i eat meat, so I'm a perfect example)
The important thing is trying to remain civil, and looking for a discussion, rather than trying to dunk on one idea or the other (this in particular to vegans too)