TL;DR

The onus was on the guests to communicate their displeasure. They didn’t and then blew up about it out of the blue. You are NTA.

NTA 100%.

I have celiacs disease (Also semi-strict ex-vegan of 11 years.) and I would not in a million years behave the way this guest couple did.

If I’m hanging out with people it’s because I enjoy their company not due to some outdated host-guest ritual in which I expect to be treated like royalty. Yes, going to a dinner party for a year and not mentioning that you would like to be included in the food— maybe offer to bring a dish for everyone and ask if they can reciprocate, plus getting pissy about it out of the blue- is beyond entitled. The guests would be the asshole in this scenario.

Most of the people commenting YTA seem to be utterly ignorant of the extent of some vegans dietary restrictions. No milk, butter, honey, nothing with gravy or meat juice or anything that touched any of those things. Nothing with an animal product as even a component of a last listed ingredient. I was never quite this picky but some of my vegan friends would send a meal back to the kitchen if they found out that meat was ever cooked in it the same pan.

Cooking for special dietary restrictions is complicated. Cooking for one type of vegetarian is not the same as cooking for another. Even among vegans there is a ton of debate about what restrictions make sense and are highly subjective choices.