Section 111A. In any city or town which accepts this section or has accepted corresponding provisions of earlier law, on January first in each year every member of its regular or permanent police or fire force who has been such for at least six months shall become entitled to a vacation of not less than two weeks during such year, without loss of pay; provided that a member, who has not been such for a period of at least six months on said January first, shall be entitled to such a vacation upon the anniversary date of his appointment. Such vacation shall be granted at such time or times as in the opinion of the board or officer in charge of such force will cause the least interference with the performance of the regular work of such force. 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.