Almost all "transfers" of a single employer 401k plan moving into a PEP are Plan Mergers, not Plan Terminations. I've never had one that was a "termination".
No step-up in vesting, no "distributable event" for the prior plan. Adoption into a PEP will constitute a "restatement" of the prior plan, btw. No need to restate a plan as long as it is handled before the restatement deadline via the plan merger.
On the final Form 5500 for the single employer plan, it should indicate the name of the successor plan, EIN of the PEP Plan Sponsor (PPP). Testing is aggregated for the full year. Annual 402(g) limits still apply (no "double dipping" in deferrals....).
Lots of moving parts in a PEP. Proceed with caution if you haven't done any.