In a large frying pan over high heat, bring 8 cups water to a boil. Season with salt. Add the prawns and boil for 30 seconds. Turn off the heat and let the prawns sit in the water for 10 minutes. Remove the prawns with a slotted spoon and place in a bowl. Add the tequila, stir together, and let sit in the refrigerator for 2 hours.
In a bowl, stir together the fish broth, tomatoes, bell pepper, onion, cucumber, 5 tablespoons of the vinegar, garlic, tomato juice, olive oil, and bread. In batches, process in a blender on high speed until very smooth, 3 to 4 minutes per batch. Strain through a coarse strainer. Season with salt, pepper, and the additional 1 tablespoon vinegar, if desired. Cover and chill for 1 hour.
To make the garnishes, preheat the oven to 400°F. Heat the olive oil and butter in a small frying pan over medium heat. Add the garlic and cook, stirring, until golden brown. Remove the garlic and discard. Place the torn bread pieces on a baking sheet and drizzle with the garlic-infused olive oil. Bake in the oven until golden and crispy, 8 to 12 minutes. Let cool.
Ladle the soup into chilled bowls. Drain the prawns, reserving the tequila. Divide the prawns among the bowls. Garnish with the cucumber, bell pepper, tomato, red onion, and croutons. Drizzle a teaspoon of the tequila on the top of each bowl of soup and serve.