The Virginia Retirement System may enter into an agreement with any political subdivision of the Commonwealth which has a defined benefit plan that is not supplemental to the Retirement System to permit any vested member of the political subdivision’s plan, upon entering service in a covered position and filing a written application with the Retirement System, to purchase service credit, of an amount to be determined by the Board of Trustees, in the Retirement System. The purchase shall be accomplished by and upon the transfer of assets to the Retirement System from the political subdivision’s plan as provided in the agreement.