Is it acceptable to calculate vesting from date of eligibility rather than date of hire?

Generally, all years of service with an employer must be counted for vesting purposes.

Two of the main exceptions are:

1) Years before age 18;
2) Years before the effective date of the plan (assuming no successor plan rules apply);

The plan document will contain the pertinent language.

Excluding years before eligibility date would not meet the vesting requirements.