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401(k) not fully vested at time of acquisition
Probably not. If you were at a small company and asked such a question, you'd get advice and links to erisa or other case law, etc.  it's safe to say that a Fortune 500 company such as IBM is going to have their facts in order, and not going to run afoul of the rules in these cases (vesting rules and takeover of other company).  I was in a company that cancelled its pension program. Those of us with the required years got the option of a lump sum payout, those with less than 5 years had no vested value and got nothing. One month longer employment, in the case of a particular coworker, would have given him a lump sum worth nearly 6 months pay.