The problem with this suggestion is that it probably could be done, but the sponsor does not want to do it. Nothing to stop the sponsor from amending the plan to make matches 100% vested immediately. The key point is that the plan administrator gets to refuse categorically to give this employee, however disgruntled, anything to which he or she is not already entitled, even if he or she was able to find an attorney willing to threaten them. Perhaps the plan administrator should ask the attorney to provide a cite that would give the client's demands any legitimacy. Threats of litigation don't, by themselves, create a reason to give in. The suit, if filed, would have to be against the plan, not the employer (who has no liability for benefit claims against the plan, after all), and there are serious consequences if a plan needlessly gives in and pays amounts to the participant to which the participant is not entitled.