"I shall embrace the day, crap lasagne and all."
You have said that you are BU, but unless you're eating it, I don't see why you are bothered, he isn't your only guest, it means that you can cook in any manner you want to.
I am Vegan at home. I will drop to Vegaterian to celebrate someone's Birthday, but I am still very choosy about what I pick. I won't eat any animal product that isn't Free Range etc.
I find that Vegan ready made foods are far superior than meat based cheap ready meals, so what he is choosing to eat May not be "crap".
I am taking my own Christmas dinner to my Daughters, I am having to possibly drink non Vegan alcohol and the odd food.
If I go over a certain quantity/quality then I spend at least a day on the toilet.
I don't think that people should be forced to drop their Ethics or health, when avoiding do both is so easily avoided by taking your own food.
I think celebrations are about getting together and everyone enjoying themselves, including what they are eating and drinking.
You can't win, as a Vege, your either making unreason demands, or are being insulting by thinking you are not being a bother by taking your own food.