Many people don't realize that the chicken broth in the Acorn Squash Puree and marshmallows atop the sweet potatoes make those dishes non-vegetarian. It's helpful to say what you do eat, as well as what you don't. Many 'traditional' Thanksgiving dishes are easily adapted for a vegetarian palate. It might be a good idea to eat something before you get to your destination, just in case you find yourself confronted with cheese balls rolled in bacon bits as an appetizer and not one all-veggie dish on the menu. It's fine to ask what's in a particular dish and politely decline something that doesn't fit your lifestyle, but now isn't really the time to explain how turkeys are raised and subsequently killed for our annual Thanksgiving feast.
Many vegetarians find a concerned host frequently asks if they've had enough to eat.
It's really quite easy to feed a vegetarian. First of all, ask them what they do and don't eat. Vegetarians fall into many different categories - I know a few who eat fish and still consider themselves vegetarians. No matter what kind of vegetarian you'll be hosting, they won't be eating the roasted turkey or ham. Jello is out, too.