Being vested in the 401k is a big deal. Really that's the main employee retention program that exists. I was laid off two jobs ago about 2 months before I hit full vesting in the 401k, which I don't think was accidental. I was still 80% vested, though, so most of the way there on the matching.
I started getting recruited for this job a couple months before hitting full vesting at the last job, and got my offer 2 weeks before I hit the full vesting point. I said "Ok, I accept your offer, but I have to work here another two weeks before I can turn in my notice." The current company was very understanding.
Full vesting + 1 day, check the 401k, "Hey boss, got a minute?"