You can use the optional GnuCash feature called Vendor Jobs to associate bills which fall under a single vendor. This is useful for when you have multiple "jobs" for the same vendor, and would like to view all the bills for one job.
To use vendor jobs, you must create them using the Business -> Vendors -> New Jobs application. Once there, you will see the New Job window. The editable fields are:
Job Info - Job Number (optional) - enter the number for this job. If left blank, a reasonable number will be chosen for you.
Job Info - Job Name - the name you want to call this job.
Owner Info - Vendor - the vendor associated with this job.
Owner Info - Billing ID - the vendor's reference to this job (e.g. their PO Number). It's the "Billing Identification" that they require on your invoices to them.
Job Active - toggles this job being active or not. This is useful to use when you have lots of inactive jobs, it is easier to find all the active ones.
New Vendor Job
To find a preexisting vendor job, use the Business -> Vendors -> Find Job application. From the results of the search, you can select a job to edit, or view.