This also depends somewhat on what it means for someone to leave the company. This is actually not a simple issue - e.g. as the CEO of the company, you may be e

This also depends somewhat on what it means for someone to leave the company. This is actually not a simple issue - e.g. as the CEO of the company, you may be entitled to fire someone, even a founder, and that stops the vesting. But usually you'll have a board of directors, and the board is allowed to overrule the CEO, and guess who else sits on the board? The co-founder. This gets very complicated very quickly. That's why you should consult lawyers (or legal documents available online, but unless you're a lawyers you should consult one anyway). I'm actually not sure how this gets resolved in most startups. Oftentimes there are clauses like Shotgun clauses that try to solve this in a catch-all manner, providing a way of forcing people to sell their shares.