Meals that are getting served.

I have two people who are Gluten free, I spoke to my caterer and let them know and they will prepare meals for them with no additional cost, I would speak to your caterer and see, if it's just one meal should be doable. These days you kind of have to cater to people's dietary needs.

You should always have a vegetarian option on your menu, regardless if it's only for one guest or 100. Most caterers will offer a proper vegetarian entree (and please don't just serve the sides and call it a day). People are paying to attend your wedding, whether it be through gifts or just their own expenses to attend (clothes, travel) so the least you can do is offer a proper meal for them.

We are doing a buffet style dinner, and have a couple of vegans coming to our wedding. Our venue offers the option to pay a little extra and get separate full plates for them. If you are doing catering of any kind then that should be a request that can be accommodated. Don't change your entire menu.

IMO I wouldn't change the entire menu for one person, I suggest asking what vegan options are available from your caterer.