A plan terminates a large group of its employees, enough to constitute a partial plan termination and 100% vesting. 2 to 8 months later, the employer hires some of these terminated participants back. Do they return to service 100% vested?

I believe they would since vesting is a protected benefit. However, I have never had a partial plan termination where employees were later rehired prior to distribution of any assets.

Any help would be appreciated.

I would treat the rehires as 100% vested. Partial termination did trigger 100% vesting on those participants. It does not matter if they did not withdraw their account balance. Those participants affected by the partial term are still 100% vested.

Must they also be 100% vested in any new contributions made to them?

Yes.

All those participants would now be 100% vested including new contributions.

I understand a partial plan termination to make the benefits accrued up to the termination date nonforfeitable. This is not the same as making someone 100% vested for life.

I would look for more confirmation of UKH's position if that is the way you want to go.

I am just being conservative in my approach to keep all contributions 100% vested. I am not sure how the IRS would look at it in case of an audit. Hence in my opinion it is better to leave the participants 100% vested for all contributions.

UKH - Your conservative approach may violate plan document terms. You cannot just vest these rehires at 100% on future contributions because they were an affected participant under the partial plan termination. The only benefits which require full vesting are those which were in their accounts when they were laid off. Any future benefits will need to vest according to the document's vesting schedule for that specific money type.

However, if the document contains language which would allow the continuance of the full vesting then I guess my previous statement is a moot point.