OK - this is the ULTIMATE thing to get a cog off. I only drew like half of it since I'm not that good at drawing anymore.

Anyways, it's an upside down L shape of about 1/2" thick metal. One part goes in the vice. Two bolts thread through the top part that have nuts (not shown) to help hold the cog in place when you're first putting it in place. A total of four pins stuck up about 1/4" from the top, and fit around the teeth, totally locking down the wheel.

Put it in the vice, turn the wheel, and you'll probably never ever have a cog that won't come off again.

I wish I had a vice and knew where to buy one. At least I know what LBS to find one at.