I can help here!
For a main, I'd suggest a Tofurkey roast. They're super easy, extremely delicious, and should be showing up in the frozen section of grocery stores pretty soon here. (They don't exist most of the year, which is a little tragic.) There are other brands of vegan roasts, and they're fine, but Tofurkey is the best, in my opinion. Also, one Tofurkey roast is nowhere near as large as a turkey, so depending on how many people are coming you might want to get two. And then you're more likely to have leftovers, which is always a good thing!
Stuffing is also easy: simply find a box of stuffing that's vegan, there should be at least one choice at the grocery store. Watch out for chicken broth and hidden dairy, I've seen both of those show up in ingredient lists and it's annoying.
Mashed potatoes and gravy are easy. You can just make them as normal, substituting plant milk and plant butter in the potatoes and veggie broth for the gravy. Some roasts actually come with gravy, but it's never as much as I want, so if you end up with something like that I suggest using that as a base and stretching with more broth and flour.
For sides, I'd recommend brussels sprouts however you want to do them. I usually go roasted with some olive oil, soy sauce, and herbs. Cranberries are also easy, I usually get a bag, either frozen or fresh, whatever I can find, then cook with a splash of lemon juice and sugar to taste. Green beans are a classic, again roasted with olive oil, soy sauce, and herbs. Really, any roasted veggie is nice.
I did a green bean and mushroom soup casserole one year, which was absolutely amazing, but took a lot of work because you have to make your own mushroom soup since Campbell's Cream of Mushroom has dairy in it. If you feel like going all out though, it's pretty worth it.
I'm not a big dessert person, so usually I don't make one. That being said, a pumpkin pie (or squash or sweet potato or what have you) should be pretty easy. Graham cracker crusts work fine and are usually vegan (I think, maybe double check) and making the filling should be straightforward. Depending on where you shop you might even be able to find some vegan whipped cream if you're into it. You can also maybe buy a dessert. I know Daiya has cheesecakes that are actually pretty good, but they're rather expensive and a bit small, so your mileage may vary there.
Good luck! Vegan thanksgiving is actually much easier than it sounds at first! And the roasts are so, so much easier to cook than a dead bird, you really come out ahead there!