Place frozen bag of soup into a large stockpot. Cover with water and heat to boiling. Cook for 35 minutes or until contents of bag is hot.
Meanwhile, heat a countertop grill or stovetop griddle over medium heat. Prepare sandwiches by layering sliced mozzarella, tomatoes, basil leaves, and optional balsamic vinegar on top of bread.
Grill or press sandwiches for 3 - 5 minutes (flip and cook on other side as well if using a stovetop griddle). Allow sandwiches to cool slightly.
Carefully remove bag from boiling water in stockpot using tongs. Allow soup to set for 5 minutes. Carefully cut open bag and divide hot soup among four bowls. Sprinkle croutons over soup. Serve each bowl alongside caprese panini sandwiches.