Lots of great ideas here but I do wonder. Not everyone has the money to go out for a really great meal at the drop of a hat. Even a really nice take away is not in some people's budget unless they save up.
The OP should not need to go out and supply her own food for a meal.
Imagine if that became a thing, then OP would have the disadvantage of being slighted by her MIL, unsupported by her SIL and, most significantly of all, her dh, and still need to buy her own food for a meal she was invited to!
I know there is not only one answer to any problem but I think, in light of what has already happened, Mermaid you need to speak to DH before even you forget how unpleasant this felt.
Tell you MIL by email/letter/in person (or phone if that is the only option), how rude you found this.
Next time you are invited at a meal time, if you choose to accept, call beforehand to see what is on offer for you, or do not accept offers of visits at 'mealtimes'!
And when there, personally, I do think it is best to accept what is offered (if veggie), even if it is a measly offering; because to not do so, plays into her hands. You can still complain about feeling slighted and left out but you can do it with something in your tummy!
I know no contact is always an option when families fall out, and if she were being really terrible, or you had kids who were being treated disproportionately to your SIL's child, I would definitely consider it. But in this situation, I would not suggest no contact straight away. For lots of reasons:
I don't think your dh would support you
I think it may even be what your MIL wants, (which I would be loath to give her!)
and you could find yourself feeling even more on the outside (as could dh)
Please tackle this, pick your battles. This is a real tangible and discernible problem.
My first thought was to make a big fuss and draw attention to MIL's cruelty, but that could so easily backfire and end up with MIL in tears "I do try to do my best," she says while stuffing a pork pie in her mouth, and you look the meanie with others telling you to be nicer to MIL! Could happen.
So always keep your cool, be honest and fair, but be assertive.
All the pay back of cooking her a veggie meal, limiting food when she visits, making fun of her etc on Facebook, these will not resolve the issues, tempting as they may be!
I think you need her to either admit "I don't like you." And you go from there, working with your dh to break this all down, before N/C - or - you make her change her behavior because she realizes she is showing herself up, big time! IMVHO