In a medium bowl combine balsamic vinegar, rice vinegar, shallots, garlic, oregano, basil, rosemary, sugar, salt and pepper. Add vegetable oil and olive oil in a thin, steady stream, whisking constantly until combined.
Use immediately or cover and chill for up to 3 days before using. If chilled, let stand at room temperature for 30 minutes; whisk before using. Makes about 3 cups (24, 2-tablespoon servings).