Preheat a grill to medium high. Lightly oil the grill grates. Combine the shallot and vinegar in a small bowl; let sit 5 minutes. Add the cilantro, cumin, oregano, red pepper flakes, 1/2 teaspoon salt and a few grinds of pepper and stir to dissolve the salt. Stir in 3 tablespoons olive oil.
Toss the shrimp and Cubanelles with 1 tablespoon of the cilantro sauce and 1 tablespoon olive oil; season with salt and pepper. Thread onto 8 long skewers. Brush the plantains with the remaining 1 tablespoon olive oil.
Arrange the plantains and shrimp skewers on the grill. Cook the plantains until lightly charred, 2 to 3 minutes per side; cook the shrimp skewers until opaque, 3 minutes per side. Cut the plantains into bite-size chunks. Divide the shrimp skewers and plantains among plates. Drizzle the skewers with the remaining cilantro sauce.