@YouJustFoldItIn, I'm afraid I'm firmly in the 'Oh god not bloody goat's cheese' camp. I'll add 'oh god not butternut squash or mushrooms and definitely not risotto' too
So what do you like for a special occasion then? Lentils? Beetroot? Mixed beans? Tofu? Only the vegetarian I most often cook for loathes tofu. But she loves mushrooms, She loves cheese and is very happy to eat anything in 'en croute' which is another thing that has been sneered at on here.
We are not mind readers and if we discounted every type of can't veggie food on the basis that lots of people might not like it you'd end up with a plain jacket potato or a plate of plain spaghetti. We are merely offering ideas that will work alongside a roast that the OP's guest might like.
I would not expect anything fancy like you suggest, and you mean well but meat eaters occasionally eat "vegetarian food" and think that's what vegetarians like.
Well I don't speak for all meat eaters nor do I think like all meat eaters, so I am not sure why you think you can speak for all vegetarians and assume that you think like all of them. Clearly you don't all like and dislike the same things as my friend can attest.
So what would make you happy on Christmas Day then, that's not 'fancy or complicated'? A tin of baked beans to have with the OP's root mash? A Sainsbury's Margerita pizza? A few Linda McCartney sausages thown in as an afterthought? Because that's not very Christmassy, it's the sort of thing you'd eat off a tray on your lap on a Tuesday after work, but if you are happy with that then so be it.
But for everyone like you there'll be another vegetarian bitching that the OP didn't make any effort and treated her like an inconvenience by serving boring, second rate food straight out of a packet.