Combine peanuts, sugar, syrup, and salt in a 2 -1/2 quart microwave safe container. 700 watt oven: Microwave 4 minutes on HIGH POWER; stir, microwave 4 minutes longer on HIGH POWER. Stir in butter and vanilla. Microwave 2 minutes longer on HIGH POWER. 1200 watt oven: Microwave 4 minutes on 60 PERCENT POWER; stir, microwave 4 minutes longer on 60 PERCENT POWER. Stir in butter and vanilla. Microwave 1 minute longer on 60 PERCENT POWER. ***Add baking soda and quickly stir until light and foamy. Immediately pour onto lightly greased baking sheet; spread to ¼ inch thickness. When cool, break into pieces. Store in airtight container. ***Note: Any cooking time adjustments necessary for lower or higher watt ovens must be made BEFORE adding baking soda.