When executing a suitescript that makes use of record.attach function to attach a file to a Vendor Bill record, what permissions are required for the script to execute the record.attach function?

don't think you need permissions for this... the file should already be in the filecabinet right? you just need the fileObjectid and the vendorbillrecid

var attachedfileid = record.attach({
                record: {
                    type: 'file',
                    id: fileObjectid
                },
                to: {
                    type: 'vendorbill',
                    id: vendorbillRecid
                }
            });

Well it's not working currently. We can upload the file to the file cabinet using the file.create command, but the subsequent record.attach fails with INSUFFICIENT_PERMISSION - You do not have permission to perform this operation error

no worries