1. Heat a grill pan over medium-high heat, or preheat a broiler.
2. Lightly brush 1 side of each bread slice with 1 tablespoon olive oil, and sprinkle with rosemary, sea salt, and pepper; toast lightly, about 2 minutes per side. Loosely cover toast with aluminum foil to keep warm.
3. Fill a large skillet with water to within 2 inches; bring to a boil. Add vinegar and 1/4 teaspoon salt, and reduce heat to low.
4. Break 1 egg into a small bowl or measuring cup, taking care not to break the yolk. Turn egg out of the bowl into skillet quickly so the egg keeps its shape when it hits water. Repeat with remaining eggs. Cook 5 minutes or until whites set and a thin translucent film forms over the yolk. (For a firmer yolk, cook about 1 minute longer.) Use a slotted spoon to gently lift eggs out of water, and place them on a paper towel to drain; cover loosely with foil to keep warm. Remove any loose whites from rim of the eggs.
5. Put a slice of bruschetta on each plate, and top with a slice of prosciutto and a poached egg. Drizzle 1 or 2 tablespoons Warm Mustard Vinaigrette over the top of each, and serve immediately.