use the 'bus driver', forget chainwhips or anything else. Rotafixing has a great chance of scratching your frame - or worse.
Wood blocks in a vise will usually allow the teeth of the cog to chew into the wood and split them most effectively.
Method - remove wheel, wrap cog in about 3 layers of old cloth to protect the sprocket (an old T-shirt is good). Lay wheel face down in vise, clamp cloth and sprocket tightly. Then tighten it again.
Clasp wheel (tyre, actually) firmly with both hands, on either side of the wheel to give maximum leverage, then turn the wheel anti clockwise (think bus driver) until you feel the threads 'give'. Voila. Unwrap cloth.
Always grease the threads of your hub/sprocket