1. In a 4- to 5-quart pan over high heat, bring about 2 quarts water to a boil. Add shrimp, cover, and cook just until opaque in thickest part (cut to test), about 2 minutes. Drain.
2. In a bowl, mix oil, vinegar, garlic, capers, and shrimp. Cover and chill, stirring occasionally, at least 30 minutes or up to 1 day.
3. Trim tips from ends of baguette and cut remaining bread crosswise into 36 equal slices. Set slices on racks on a 14- by 17-inch baking sheet.
4. Bake in a 425° oven until golden, 6 to 8 minutes. If making ahead, store cool slices airtight up to 4 hours.
5. Seed and finely chop tomato. Add tomato, bell pepper, and onion to shrimp mixture. Season with salt to taste.
6. Spread cheese on toast. Place vegetables and 1 shrimp on each piece.
Nutritional analysis per piece.