In a small nonreactive bowl, whisk together the vinegar, capers, garlic, mustard, salt, and pepper. Add the olive oil and whisk to combine. Set the vinaigrette aside while you assemble the remaining sandwich ingredients.
Using a serrated bread knife, cut each sandwich roll in half lengthwise. If the bread is very dense and doughy, use your fingers to pull out some of the doughy interior, leaving a 1/2-inch shell on both the top and the bottom portions of each roll.
Drizzle 2 tablespoons of the vinaigrette over the insides of each roll, 1 tablespoon for the top and 1 for the bottom. Divide the tuna evenly among the bottom portions of each roll. Divide the tomato slices evenly over the tuna. Divide the bell pepper slices evenly over the tomato slices, and then divide the olives evenly among the sandwiches.
In a medium mixing bowl, toss the mixed greens with 1 tablespoon of the remaining vinaigrette and top each sandwich with some of the greens. Drizzle any remaining vinaigrette and scatter the basil leaves over the sandwiches and place each sandwich top over the filling. Press down on each sandwich with your hands or with a large spatula to compress slightly.
Cut each sandwich in half and serve, or wrap in plastic wrap and refrigerate for up to 6 hours before serving.
Note: This sandwich could also be done on one long baguette and then sliced into serving pieces.
Yield: Four 6-inch sandwiches