>>fnbr+(OP)
What a lot of people seem to be missing here is that RSUs are usually double-trigger for private companies.
Vested shares are not yours. They are just an entitlement for you to be distributed common stock by the company. You don't own any real stock until those RSUs are released (typically from a liquidity event like an IPO).
Companies can cancel your vested equity for any reason. Read your employment contract carefully. For example, most RSU grants have a 7 year expiration. Even for shares that are vested, regardless of whether you leave the company or not, if 7 years have elapsed since they were granted, they are now worthless.