In a small saucepan, combine the vinegar, brown sugar, garlic, and olive oil. Bring to a simmer over medium heat, then reduce the heat to medium-low and cook, stirring occasionally, until the mixture is reduced by half and thick enough to coat the back of a spoon, 6 to 8 minutes. Set aside to cool, 10 minutes.
On a serving platter, arrange the mozzarella, tomatoes, and basil. Drizzle with the cooled dressing; season with the salt and pepper.
