How many vegetarians will you have? Black bean tacos sound incredibly messy. I've had good luck just cooking TVP (texturized vegetable protein/fake ground beef) like I would normal fillings. It is much easier to eat in taco form. If it's a small gathering I wouldn't make anything special but if you have a bunch of vegetarians they might appreciate it.
If you're planning on something other than fish for the vegetarians, I would think that would be fine for the guests who don't want fish.