A board of five directors shall be elected at the same election held for the establishment of such district, and the ballot therefor may have printed thereon the names of such candidates, or the voter may write upon his ballot the names of the persons voted for as directors; and the five persons receiving the highest number of votes so cast shall be the directors of said district, and shall hold office until the next regular election. In case of vacancy in said board, or if the number of directors is reduced by any cause to less than three, said vacancies shall be filled in the same manner as provided by law in such cases for directors of water improvement districts under Chapter 2 of this title.