Is cast iron skillet suitable for vegans?
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.