The board may establish a retirement system for the officers and employees of the VTA and provide for the payment of annuities, pensions, retirement allowances, disability payments, and death benefits or any of them. The adoption, terms, and conditions of any retirement system covering employees of the VTA in a bargaining unit represented by a labor organization shall be pursuant to a collective bargaining agreement between the labor organization and the VTA. For purposes of this section, “officers” does not include members of the board of directors.