If a plan merges with an existing plan and as a result are accepting a better vesting schedule, what vesting schedule would any terminated employees be required to follow? Would they follow the schedule they were on at the time of the termination, or would they advance to the new schedule?

Likely the "old plan" will state that vesting is determined under the terms of the plan as of the event date (death, retirement, disability, severance, etc.). Later plan provisions probably will not impact prior participants unless the plan change includes a retroactive effective date or other specific coverage.

In addition, many plan amendments will include language noting that its provisions apply to participants who have at least one hour of service after date X.

So, what does your plan say?