As C.B. Zeller implies, you have to read the plan document because there is a small chance that your plan contains a 5-year suspense account provision instead of a cash-out and buyback provision, in which case you would have to vest the suspense account.
Also, you do not provide a full description of potentially important facts. If the individuals were a substantial portion of the workforce and they terminated in connection with a winding down or shrinking of your business, you could have what is called a "partial termination" that would require full veting.