When a partial termination appears to have occurred, it is always a good idea to cover vesting through an amendment, since otherwise it is not necessarily clear that even those displaced have been fully vested.
Note that when a defined benefit plan freezes all accruals, that could be considered a partial termination. To freeze benefit accruals, of course, there must always be a plan amendment. When freezing accruals, it is a really good idea to freeze plan entry, and since the freeze could be a partial termination, it is also a really good idea to grant full vesting to all those still employed on the freeze date. Some provisions concerning rehires may also be a good idea, so if a participant who had terminated before the freeze is later rehired, it will be clear what is to be done. For example, if someone had terminated with 40% vesting, was not active on the freeze date, and was subsequently rehired, what will be done with respect to the 60% of the pre-break accrued benefit that was not vested?