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