you could not reflect it as an expense. if you didn't pay it and had no obligation to pay it, then there is no need to record it. If it was your obligation to pay, then I would get something in writing (email) from the 3rd party stating that they will pay it and that you owe no obligation to them regarding those funds.
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.