Place 1/2 cup of the balsamic vinegar in a small pot. Bring to a boil and reduce by half. Cool to room temperature, and then reserve. Process the remaining ingredients, except whipping cream and herb sprigs, in a blender or food processor until smooth. Chill soup well. Adjust seasoning before ladling in chilled soup bowls. Drizzle top with reduced vinegar and whipping cream. Garnish with mint or basil sprigs. Serve immediately.