Vendor payments tied to project

Warning--I'm not a CPA. We bill a customer for services rendered and use a vendor to provide those services. If we use a project record for this (we're not now) and make the vendor a project resource, how will the payment to the vendor be shown in the project? The vendor will send us a bill for services performed and we pay them--they don't track their time in the vendor center. Can that payment to the vendor be tracked to the project in some kind of project profitability report? I guess I could create a custom field in the bill