I would suggest using international standards such as ISO 26000, or GRI (Global Reporting Initiative). Indeed in your case (services) it is very important to take into account the specific CSR challenges of the various spend categories, as Sustainability issues are very different between those categories (ex: Print, Temp Labor, Facility Management, Logistics, IT,...)
If you are looking for a solution, EcoVadis, an Ariba partner, is operating a "Suppliers Sustainability Rating" database, which can be integrated into Ariba. Our database covers assesment of vendors on 21 CSR criteria, taking into account the specific regulations/issues/labels applicable to 150 spend categories. Our collaborative platform is used by 30 Global 500 companies to access to Sustainability ratings of suppliers across 90 countries.
Please do not hesitate to contact me (pfthaler at ecovadis dot com) should you wish more information, on how you could structure your "Vendor CSR rating system"