This is an interesting idea — variable vesting based on milestones. It would be a great way to reduce risk, though of course there are issues that could arise that would make this complicated. For example, if the technical person vests based on hitting certain product-related goals, does he get penalized if part of the dev team gets hired by another company, which makes it harder for him to hit the milestones? Or for the non-technical cofounder, if the economy goes into a tailspin, and it's much harder to make sales than the cofounders anticipated, should he not vest, even if he's doing everything he can?

But overall, I think this is a really intriguing idea. Points could be assigned for what each founder is bringing to the table (concrete idea, market research, cash), and they would get some vested stock for that. Then the rest of the founder's shares could be put into a pool and vested to particular founders that hit their milestones. You wouldn't want to make it too adversarial, but it's possible the benefits of incentive alignment would overcome the downsides of the zero-sum game thinking that the system could engender.