Let me give you a real data point of what happened at a leading social network company. One of the early hires was fired after a few months on the job, and the company agreed to not only vest him, but FULLY ACCELERATED, 4 years. Of course lawyers drew up non-disparagement agreements and such. This actually happened multiple times over the course to IPO, but it was a smart move in hindsight to ensure there was no leaks and legal battles that could cause huge problems later.