Since the vesting is done you own your shares and could work zero hours and still be entitled to them. The hard work you did over the 5 years as they vested means something.

But, if you want your co-founder to like his situation and work hard to make the company keep going I see this as like doing another round.

You need to raise X and valuation Y and dilute yourself accordingly. i.e. the company is in crisis because one of the co-founders is sort of leaving. To make up for that, raise some capital. Figure out the amount needed to raise and then work backwards from that number.