If I had to cater for a vegetarian for Christmas lunch, I'd consult them to find out what they would prefer, but without a firm steer, I'd go for making all the vegetable dishes vegetarian (not a problem here, as I don't roast my potatoes in goosefat or add bacon to the sprouts).
Main dish - it would have to be something that goes with gravy, stuffing*, bread sauce, cranberry sauce, Brussels sprouts, carrots, peas, roast parsnips, roast potatoes, as that's what the rest of us are having.
One obvious way is a fake meat approach - Quorn roast, the Linda McCartney fake beef someone mentioned. I could do some vegetarian sausages to be the equivalent of the cocktail sausages I do with the turkey. Not everybody likes or approves of fake meat, though.
The other way, without involving nuts or mushrooms, would have to be either a fancy vegetable dish (squash-based, maybe, and definitely not strongly spiced or involving tomatoes, which I think would clash horribly with the traditional Christmas vegetables and gravy) or a loaf/roast type thing (or a casserole) based on lentils or other pulses. I like the idea of adding in things I don't normally do for us - Yorkshire pudding, cauliflower cheese - if the main event is a fancy veg dish, as that ups the protein content.
I love pastry but on top of all the other rich food is it perhaps a bit much?
I'm in the fortunate position, though, of having two ovens, a 6-burner hob and a microwave, which is a big help! With less oven/hob space, not all of this would be practical. It would need a lot of planning to get everything done and kept hot to be served together.
*No way would I forego my lovely turkey gravy for an instant Bisto version, but I'd happily make a vegetarian gravy separately.
**It wouldn't be a big deal to make up some stuffing that doesn't contain any suet, sausagemeat etc.
Good luck!