You can always exclude any portion of compensation for HCEs and still have a safe harbor definition of compensation. That said, this this phantom stock plan sounds like a nonqualified deferred compensation plan. Here is what 1.415(c)-2(c) says:
As support to Larry's comment, the original questioner may benefit by reading IRC 411 (Minimum Vesting Standards). Specifically, see the first sentence of 411(a), and then see the definition in paragraph (8). https://www.law.cornell.edu/uscode/text/26/411
Of course they are 100% vested; you said that in your message. They reached NRA while working, your plan fully vests at NRA. What is the issue? "Will they STILL be 100% vested"; if they ARE 100% vested, they remain so. Did you mean to ask "Do they BECOME 100% vested"? And the answer is still YES.
Does that help at all?