The fish tacos at the TacoBelly "pop-up" in PB Steak are very good. Same for the Baja style shrimp tacos on the lunch menu at Eating House in Coral Gables. I've not been in ages but the Baja style fish tacos at Mercadito were also good, and they probably have a couple other veg / fish options available too. Another I haven't had in ages but recall being good are the Baja style fish tacos from the Jefe's Original truck.
Haven't tried yet but I'd also check out Hua Hua's on South Beach.
Pretty sure that Viva Mexico (f/k/a Con Sabor a Mexico) will not have fish or veg options. Here's a shot of the menu:
Nacho Bizness is a food truck that does amazing Cali style tacos. Check their Facebook page for locations on specific days but they are normally in dowtown Ft Lauderdale Wednesday and Friday lunch 1 block north of Las Olas in the parking lot behind the Bank of America building.
Mi rinconcito Mexicano has what I consider an authentic carne asada tacos. the also have a authentic Mexican bakery.
For fish tacos try the raw tuna taco from my ceviche
Mi Rinconcito Mexicano
1961 SW 8th St, Miami, FL 33135
235 Washington Ave, Miami Beach, FL
1250 S Miami Ave, Miami, FL