Lay piece of parchment paper on heat resistant counter top or grease a baking sheet.
Mix sugar and karo and cook in microwave for about 4 minutes, or just until amber colored.
Remove from oven (it's very hot, be very careful). Add peanuts, cook about l to 2 minutes more.
Remove from oven, then add the margarine, baking soda, and vanilla all at once, and mix thoroughly. Spread on parchment paper, (this sets up quickly). Cool, then break into pieces and put in jar, with tight fitting lid.
My microwave is 1000 watts, so I really don't leave my brittle in too long. I make this all the time, it is very easy and delicious. I usually take the margarine, baking soda, and vanilla and put it together in a bowl, so I can work quickly with it.