Well, for starters, you shouldn't really post these kinds of details on Hacker News, your cofounder probably reads it and may be pretty pissed that you turn to

Well, for starters, you shouldn't really post these kinds of details on Hacker News, your cofounder probably reads it and may be pretty pissed that you turn to the community to discuss what you should be doing regarding the finance of the company and his lack of work while he was diagnosed with a lethal illness. Then, it's pretty obvious that you should take this as a problem that "hits the company", as in you should solidarly take the "loss" that his illness is causing the company. So he should be paid the same as if he was fit to work, and vest him like you are.