Vegan Thanksgiving?

First of all, you are so kind to accommodate your vegan guests! As a vegan myself, I appreciate any vegan dish that someone goes out of there way to make for me! Very thoughtful of you.
Mashed potatoes and gravy is super easy to make vegan and really tasty!
For the potatoes just substitute the butter for Earth Balance Vegan Buttery Spread and the milk for plain almond or soy milk. Adding sautéed garlic and some thinly chopped kale makes it super devine!
You can make gravy using mushrooms, flour, earth balance, and some spices.
I made cornmeal crusted Brussels Sprouts last year. They turned out crazy good! Non-vegans loved them as well as non-Brussels Sprout lovers.
You can find the recipe here:
http://boldvegan.com/vegan-spiced-brussels-sprouts-recipe/
Good Luck!
Cheers and yummy food.

Some random ideas: Quinoa pilaf, spicy spinach-mint rice w/carrots and corn, mixed lentil and rice salad, or a mixed-bean salad with a tangy dressing.

Vegan is really not hard to make. Look at what is available rather than what to avoid. Vegetables, nuts, fruits, seeds, potatoes (without milk, cheese, dairy). The suggestions above are wonderful! I recommend you make something you will enjoy eating with your vegan friends. If you love the recipe, they will too and be honored to share it with you.

I like the heartiness of these stuffed portabellos from the Vegetarian Times Thanksgiving cookbook:
http://books.google.com/books?id=LAQAAAAAMBAJ&lpg=PA40&ots=9uohJS44nv&dq=%22would%20you%20like%20a%20bite%20stuffed%20portobello%22&pg=PA40#v=onepage&q=%22would%20you%20like%20a%20bite%20stuffed%20portobello%22&f=false

As a vegan, it's always nice to have something that can pass as a main and not have just sides, although those are so yummy, too! I'm getting hungry just thinking about it.

I made the most delicious vegan winter squash recently -- roasted an unpeeled, halved butternut squash, cut side down; once roasted, I scooped out the sweet soft insides and put them in a bowl with some cashew "cream" (made by soaking 1/2 cup of cashews in water for about several hours, draining, then blending at high speed with a couple tablespoons of water), a pinch of salt, a pinch of freshly grated nutmeg and a few grinds of Malabar pepper.
I'll be making these roasted sweet potatoes this year for Thanksgiving: http://food52.com/recipes/19495_sallys_special_sweet_potatoes
Another good side would be this: http://food52.com/recipes/7836_roasted_autumn_vegetables_with_fresh_herbs
I'll be serving these Brussels Sprouts at some point over this Thanksgiving weekend, too, without the parmesan: http://food52.com/recipes/7841_perfect_brussels_sprouts. (I like these with balsamic vinegar, somewhat reduced, instead of sherry vinegar.)
;o)

Check out this blog post: http://www.happyhealthylonglife.com/happy_healthy_long_life/thanksgiving/

I love the sheperd's pie idea. And so that your guest doesn't feel singled out, I would aim for a vegan soup and salad, of which there are many delicious options. As for a main substitute, I would go for something with either chickpeas or lentils--hearty and perfect for fall. You can search for either of these ingredients here to see what strikes your fancy, but two ideas that come to mind are:
http://food52.com/recipes/8565_mujaddara_with_spiced_yogurt
http://food52.com/recipes/13889_ancient_african_spice_berbere_spiced_chickpea_yam_mango_stew

In full disclosure, I have not made the latter yet. I nonetheless think you can easily substitute your own spice blend for the berbers if need be (see headnotes to get an idea of what it's made of). And if you wanted to swap the yams for butternut squash or sweet potatoes, that would probably work too.

If you don't go with butternut squash elsewhere, this may be great too: http://food52.com/recipes/7763_kale_salad_with_roasted_butternut_squash_and_spiced_pecans

Here's a vegan shepherd's pie dish:
http://food52.com/recipes/19408_vegan_lentil_shepherds_pie_with_parsnip_and_and_potato_mash
This is delicious and can easily be made vegan: http://food52.com/recipes/2434_one_pot_kale_and_quinoa_pilaf
And also:
http://food52.com/recipes/17866_raw_kale_salad_with_lentils_and_sweet_apricot_vinaigrette
http://food52.com/recipes/18056_lemon_herb_quinoa_with_hemp_seeds_spring_peas_and_basil