In any city or town which accepts this section, all members of its regular police or fire force may be granted a vacation without loss of pay. Such vacations shall be computed in the following manner:
For five years' service, but less than ten years' service, a vacation of three weeks.
For ten years' service or more, a vacation of four weeks.
Section one hundred and eleven shall not apply to the members of the regular or permanent police or fire force in any such city or town.
Mass. Gen. Laws ch. 41, § 111D