You could try a shepherd pie type dish (just skip the dairy/use a substitute in the potato topping):
http://www.bbcgoodfood.com/recipes/10035/golden-veggie-shepherds-pie
If you need to make things ahead, I wouldn't focus on a separate vegan main dish and go with traditional Thanksgiving sides--many are vegetarian and can be made without dairy. If it's only one person who is vegan, they will still have plenty to eat.
(If most of the guests aren't vegetarian, they probably won't eat much of the main dish in any case.)