If an outsource company insists on paying other vendors itself, ask to see those vendor bills. The amazing thing about meeting planning is that your work is never done. While you are handling the internal "public relations" on site, someone must be checking on the meeting rooms, airport transfers, rooming lists, and myriad other details in the meantime. Ah, if only you could clone yourself.
While that solution isn't available--not yet, anyway--another more practical answer is to outso