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 and 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 commitment, service level agreement and escalation processes should be listed.
Vendors should at least adhere to the same (or higher) security standards as your own company. Make sure to include your requirements, standards and best practices in your vendor agreements.