Hi,
If it is material PO- you can easily do it by creating a condition type for commission(M/06) and create PO for Main vendor, and assign commission vendor to commission condition type in PO. Later you will do invoice verification separately for main vendor and commission vendor.
In case of service PO - You have above earlier said option but why your business have commission on each service which is a additional service on service!
Regards,
Biju K