If the vacancy is to be filled at a general election and shall occur before the primary election to nominate candidates to be voted for at such general election, candidates to fill the vacancy shall be nominated at such primary election in accordance with the time requirements and the provisions and procedures prescribed in article five of this chapter. When nominations to fill such vacancy cannot be so accomplished at such primary election, and in all cases wherein the vacancy is to be filled at a special election, candidates to be voted for at such general or special elections shall be nominated by a state convention to be called, convened and held under the resolutions, rules and regulations of the political party executive committees of the state. The laws prescribing the manner of calling, constituting and holding conventions to nominate candidates for presidential electors shall, insofar as applicable, govern conventions to nominate candidates to fill any vacancy in any office to be filled by the voters of the state as a whole, except that, in lieu of the magisterial district conventions in the several counties, the county executive committee shall call and convene a county convention at the county seat with delegates thereto apportioned to and representative of the several magisterial districts of the county as provided in section twenty-one of article five of this chapter. The county convention shall proceed to select the county's prescribed number of state convention delegates from the several magisterial districts thereof and the chairman and secretary of the convention shall promptly certify the names and addresses of the persons so selected as delegates to the state convention to the chairman of the state executive committee of the political party.