I've been bringing my own plate of food to family dinners for years now, I consider myself a professional!

This year I get to host a second night Seder, this will be the first time I get to eat something other than matzo and salad at a Seder so I'm really excited about that.
Depends on what you want, do you want to do your own kind of thing, or do you want to make the vegan version of what everyone else is having? Do you do processed mock meat? How much time do you want to spend in the kitchen?
You could make some scalloped or mashed potatoes. You can even make a nutritional yeast sauce to make "cheezy" scalloped potatoes, or veggie mushroom gravy.
I'd bring a side veggie to share. Your family will probably have a veggie side, but it will likely include dairy if they're like my family. Asparagus is a wonderful springtime veggie and easy to prepare, just roast in the oven with a little olive oil, salt and pepper for about 15 min.
If you want to go non-traditional you could just look in some cookbooks or around the net for a main dish that sounds good to you and make that. You can buy a mock meat product at the grocery store and dress it up with homemade gravy, that's about the quickest and easiest way to go. Asian markets for some reason seem to have a really great selection of mock meat of every kind, you might even be able to find "vegan lamb". I made this
"turkey" roast last Thanksgiving. It was kind of a lot of work, but I don't mind spending time in the kitchen and it was goooood! Even my omnivore sisters and aunts liked it (the boys were all too "chicken" to try it

) Plus I had sandwich leftovers for days.
Hope that helps a little. Enjoy the gathering with your family!