I don't want my MIL to bring a cheese platter to my vegan wedding
How do you know for sure that they're bringing it to your wedding ?
I will tell them off for trying to ruin my wedding.
It is your wedding, so you get to make the rules. Try to be a bit more flexible though, as I'm sure not every attendee is following a vegan diet like yourself.
I'm not flexible, I just want people to follow my rules no matter what.
I understand it's your wedding and definitely you get to make rules. But It doesn't hurt to make an exception for someone as your MIL. She's not forcing you eat her cheese platter.