We have the vegetarian/vegan/differences of foods in my family too. I would start with a soup (I'm thinking some kind of pureed roasted vegetable soup.
I love the idea of food in ramekins because it feels fancier, and it's easy to prep ahead of time. Something like shepherd's pie can be vegan by making a mushroom gravy, using lentils or even TVP or something for protein, and adding in your veggies and baking like usual. If you're willing to make your own crust or buy a vegan crust, you can make an awesome veggie pot pie with all different root vegetables.
There is a pasta dish with roasted veggies that we make all the time and it's vegan if we omitted the cheese. Most dry pastas are vegan, so that's an easy one. Tex mex is super easy to do vegan (beans!), but I don't know how Christmassy it feels for you.
Whatever veggie sides you're having just make sure to keep them vegan, which is usually not an issue.
The dessert is always my kicker. Maybe you can just buy something. I hate baking, lol!