Season 4 salmon fillets with 1/4 teaspoon salt and 1/4 teaspoon black pepper each. Brush grill rack with vegetable oil. Place fillets on rack and cook 4 minutes on each side. Remove from grill.
Place tomato halves (cut side down), onion strips, and chili pepper halves on the oiled grill rack. Cook tomatoes 6 minutes, then flip and cook 1 minute more. Cook onions and chili 6 minutes on each side. Remove vegetables and let cool.
Coarsely chop the cooled tomatoes and onions, and finely chop the chili. In a bowl, combine tomatoes, onions, chili, 1/4 teaspoon salt, 1/4 teaspoon black pepper, and juice from half the lime.