If it's a good friend I'd try to accommodate them, but I think if you have special dietary restrictions beyond allergies -- which are really easy to accommodate -- that you should ask if it's okay to bring a salad or something in that vegans eat, rather than try to force the host to comply.

Like if you have a seafood allergy, I can work around that. If you're a vegan, well... That's harder to accommodate. I could accommodate a vegetarian better than a vegan.