I have a cast iron skillet and one of the guests coming for dinner is vegan. Since cast iron is seasoned by all kinds of grease over time I wonder if this is suitable for vegans. I don't want to offend anyone or witness some drama.
A vegan is not going to eat your pan, just the food that was made on it. As no animals were harmed in the making of your pan (well, probably but how would you know) the pan itself wouldn't be an issue. Of course if a tiny bit of pan seasoning could go into the food, however anything else used in the preparation of the meal like cutting boards could cause a small amount of meat product to go into the food as well. Your kitchen is not vegan, and if a vegan has a problem with that they should not come over for a meal.
I'd be extremely surprised if the subject of your pots and pans came up to be honest, vegans I know appreciate people going to the effort of making a vegan meal for them.
You'd have to ask your vegan to be absolutely sure. If they're practical, they'll acknowledge that there might be a bit of meat fat polymerized onto the pan but they won't be actually eating it, as long as you've seasoned and cleaned well. If however they're sufficiently strict, they could conceivably say, no, it's touching an animal product, I won't eat it.
I wouldn't call that person a common vegan (most are more practical), a reasonable vegan (they're not actually consuming animal products), or a polite guest (if you have extremely rare dietary restrictions you should communicate them clearly), but it's at least possible, so I feel it's worth pointing out since you sounded like you might be prioritizing avoiding offense over reason. You could in general safely assume she's the practical kind - I don't know any vegetarians (or vegans) who would be that picky. But if you can't ask her, there's of course no way to be totally sure, so if you want to make sure she'll eat everything and think she could be a very strict vegan, I suppose you could find another pan.
I am a vegetarian and I am totally put off by the idea of seasoned pans that don't get cleaned with soap and water. One of the reasons I am a vegetarian is because I consider meat to be unclean (not a religious objection, just my own many-years' judgment), and I won't eat out of a pan I would consider uncleaned, which seasoned pans would be to me. It isn't that I don't understand the chemistry---I do---I know the germs are destroyed by the heat. But the concept of having my food cooked in a pan that is never actually stripped of its old oil (for the sake of the iron pan's surface health and nonstickability) makes me know that seasoned pans would not be for me, even with someone else cooking for me, and I love to be cooked for. Just a weigh-in here...also, all the vegans I've ever known are wayyyy fussier than I am, and I would be surprised that they wouldn't object to that type of pan.
I've only been a vegetarian for a little over 3 years. My roommate used a cast-iron skillet with vegetarian-appropriate seasoning to fry burgers. I thought I could handle it, but when the meat was cooking, and for several hours afterward, the smell of the cooked meat was literally disgusting. I am most surprised at myself, as I always loved burgers, even miss them some times. I didn't quit eating meat because I didn't like the taste, it was specifically for compassionate reasons. I guess after 3 years, it has become repulsive.
I tried to cook with the pan after scrubbing it well, but the smell of the cooked meat rose up as soon as the pan was hot, and I knew I could not use the skillet again and went and bought a new one.
I don't care if he uses one of my non-stick pans, or forks or knives, they don't absorb the animal juices like the seasoned cast iron skillets do.
What people seem to be missing in some of these answers is that some people will become very ill upon eating trace amounts of animal material. Ask you guest if it is suitable or inform your guest of your intent so they can decide for themselves if they would like to eat your food.
There's no dishonor in asking the question.