In a 1 1/2 quart casserole stir together peanuts, sugar, syrup and salt. Cook 8 minutes on high in microwave oven stirring well after four minutes. Stir in butter and vanilla. Microwave 2 minutes longer on high. Add baking soda and quickly stir until light and foamy. Immediately pour onto lightly greased baking sheet; spread out until very thin. When cool, break into small pieces. Store in airtight container. Makes about one pound.