In a small bowl, whisk together the balsamic vinegar, brown sugar and vanilla.
In another bowl, place the strawberries, blueberries and raspberries. Pour the balsamic vinegar mixture over the berries. Let the fruit marinate for 10-15 minutes.
Refrigerate or serve immediately. To serve, divide the berries into 2 serving dishes. Place a shortbread biscuit on the side of each bowl.