This is a good use of Jobs. You would setup the Job, Job Tasks, and Job Planning Lines. The Job Planning Lines would be for a Resource, which represents the service you will be purchasing from the Vendor. You would then create a Purchase Order to the Vendor for that Resource to track the purchase of the service. When the Vendor performs the service, you record the invoice against the PO. Unfortunately, there is no automation on the Jobs side to create those Purchase Orders, so you either do it manually, write an extension, or use one of the many Jobs apps on AppSource to complete the process.
Once the Job is complete, you can either invoice your customer from the Job or if it is for internal work, you simply complete the Job.
