1. Cut grape tomatoes in half or into thirds. Place them in a bowl and toss with vinegar, olive oil and salt. Allow them to sit for at least 10 minutes, or up to an hour at room temperature.
2. Lay bread open on the counter. Place 2 pieces of prosciutto on one slice of bread. Layer on 3 slices of mozzarella, then 3-4 basil leaves. Top with half of the tomatoes.
3. Drizzle the remaining slice of bread with some of the balsamic/olive oil mixture at the bottom of the bowl. Close sandwich.
4. Sandwich can be toasted on panini press, in a skillet or eaten as is.