In a small bowl, combine chiles, garlic, vinegar, and whisk to combine. Set aside.
Heat a charcoal or gas grill to medium-high (about 450°) heat.
Cut three 2-in. diagonal slashes in the fish, approximately 2 in. apart. Using a spoon, spread shrimp paste evenly on each side of the fish, taking care to spread the paste into the slashes. Dividing equally, place lemongrass, ginger, lime, chiles, salt, and one sprig of cilantro in the cavity of each fish.
Cut banana leaves into 10- by 16-in. rectangles. Make a rectangle out of two layers of banana leaves and place 1 fish in the middle of the rectangle. Wrap the fish as you would wrap a burrito. Using kitchen twine, tie into a tight bundle. Repeat for the second fish.
Place banana leaf-wrapped fish on cooking grate. Cook until banana leaf is charred on one side, about 15 minutes. Turn fish over and cook 5 to 10 minutes more. Unwrap fish. The flesh should no longer be translucent and an instant-read thermometer inserted into the center should read 145°. Serve fish at the table on banana leaves with chile vinegar on the side.