Tell them there is a change of plan and it is now a pot luck dinner with everybody bringing a different vegetarian dish.  They'll be keen to convert you with their favourite recipes.  I've even had tofu that was so yummy Marinated and deep fried with a chillie soy and garlic sauce.   Deep fried camembert with plum sauce is good too.  Especially if they can eat cheese. And if they eat eggs there is spinach or asparagus quiche. But Id say hey guys potluck!!