The 3rd party who is paying that vendor bill on your behalf, what is the agreement with him? did he owe you some money and you are offsetting his account against the vendor bill that he will be paying? in this case, you can record the bill and increase the liability for your vendor and then on the other hand, you can issue a Debit/Credit Note offsetting your AR and AP accounts where both the receivable and payable are offset.
I hope this helps.