Grilled-Fish Tacos

1/2 cup cider vinegar

1/4 cup sugar

7 tablespoons fresh lime juice, divided

1/4 cup chopped cilantro

1 tablespoon olive oil

1 

1/4 pound (1-inch-thick) skinless mahimahi fillet or tuna steak, cut into 12 pieces

2/3 cup mayonnaise

2/3 cup crema or sour cream

12 (6-inch) corn tortillas

6 to 8 radish es

1 to 2 (6- to 8-ounce) firm-ripe avocado s

4 cups thinly sliced green cabbage (1/2 small head)

12 (6- to 8-inch) wooden skewers, soaked in water

30 minutes

Accompaniment: lime wedges