I went through this with co-founders recently. It was unpleasant and expensive, but at least everyone knows where we stand w/r/t leaving, vesting, etc.
This excerpt from Do More Faster is worth reading:
"A common reason for startup fatalities, particularly in the early days, is some sort of conflict between co-founders. One of the main reasons for co-founder conflict is that many aspects of the relationships were either ill-defined or misunderstood. To minimize the chance of this, it’s critical that you and your co-founders come to agreement on some key issues. I’ve framed the most important of these as a set of questions that the co-founders should be asking each other as they enter into the business relationship.
Many of these questions are hard but they get only harder with time. The sooner you address them, the better off your startup will be."
Not easy to bring up in the honeymoon phase of an idea that yes, we need a vesting schedule. My experience is most of the time (CPG startups in so. Cal) my co-founders don't even know what one is. There was resistance to the education process and agreement to terms but worth it.