Keep Records of Partners, Vendors, Contracts, SLA's
Have an up-to-date list of all your suppliers, vendors, contract numbers,contact info and what services they supply
Make sure this inventory is available both online as offline, so you can reach out to them for support in case of outage or service degradation.
If any vendors have (remote) access to any systems, monitoring tools or management interfaces, this information should be documented and reviewed regularly
Any contractual obligations, security commitments, service level agreement and escalation processes should be listed
Vendors should at minimum adhere to the same or higher security standards than your own company. Make sure to include your requirements, standards and best practices in your vendor agreements.