On a hot grill, place the oysters and grill until they open, 5 to 8 minutes. In a saucepan coated lightly with oil, caramelize the onions, about 3 minutes, then add the chipotle and tomatoes. Add vinegar and season. When the oysters open, be careful not to spill out the liquor, and top with warm salsa. Serve on a bed of fresh seaweed.