Section 111D. 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.